I2C-bus specification 3 - Philips Semiconductors / NXP Semiconductors

THE I2C-BUS SPECIFICATION

VERSION 2.1

JANUARY 2000

Philips Semiconductor s

The I2C-bus specification

CONTENTS

1 PREFACE. . . . . . . . . . . . . . . . . . . . . . . . . . .3

1.1 Version 1.0 - 1992 . . . . . . . . . . . . . . . . . . . . 3

1.2 Version 2.0 - 198. . . . . . . . . . . . . . . . . . . . . 3

1.3 Version 2.1 - 1999 . . . . . . . . . . . . . . . . . . . . 3

1.4 Purchase of Philips I2C-bus co mponents . . 3

2THE I

2C-BUS BENEFITS DESIGNERS

AND MANUFACTURERS. . . . . . . . . . . . . . .4

2.1 Designer benefits . . . . . . . . . . . . . . . . . . . . 4

2.2 Manufacturer benefits. . . . . . . . . . . . . . . . . 6

3 INTRODUCTION TO THE I2C-BUS

SPECIFICATION . . . . . . . . . . . . . . . . . . . . .6

4THE I

2C-BUS CONCEPT . . . . . . . . . . . . . . .6

5 GENERAL CHARACTERISTICS . . . . . . . . .8

6 BIT TRANSFER . . . . . . . . . . . . . . . . . . . . . .8

6.1 Data validity . . . . . . . . . . . . . . . . . . . . . . . . 8

6.2 START and STOP conditions. . . . . . . . . . . 9

7 TRANSFERRING DATA. . . . . . . . . . . . . . .10

7.1 Byte format . . . . . . . . . . . . . . . . . . . . . . . . 10

7.2 Acknowledge. . . . . . . . . . . . . . . . . . . . . . . 10

8 ARBITRATION AND CLOCK

GENERATION . . . . . . . . . . . . . . . . . . . . . .11

8.1 Synchronization . . . . . . . . . . . . . . . . . . . . 11

8.2 Arbitration . . . . . . . . . . . . . . . . . . . . . . . . . 12

8.3 Use of the clock synchronizing

mechanism as a handshake. . . . . . . . . . . 13

9 FORMATS WITH 7-BIT ADDRESS ES . . . .13

10 7-BIT ADDRESSING . . . . . . . . . . . . . . . . .15

10.1 Definition of bits in the first byte . . . . . . . . 15

10.1.1 General call address. . . . . . . . . . . . . . . . . 16

10.1.2 START byte . . . . . . . . . . . . . . . . . . . . . . . 17

10.1.3 CBUS compatibility. . . . . . . . . . . . . . . . . . 18

11 EXTENSIONS TO THE STANDARD-

MODE I2C-BUS SPECIFICATION . . . . . . .19

12 FAST-MODE. . . . . . . . . . . . . . . . . . . . . . . .19

13 Hs-MODE . . . . . . . . . . . . . . . . . . . . . . . . . .2 0

13.1 High speed transfer. . . . . . . . . . . . . . . . . . 20

13.2 Serial data transfer format in Hs-mode. . . 21

13.3 Switching from F/S- to Hs-mode and

back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

13.4 Hs-mode devices at lower speed modes. . 24

13.5 Mixed speed modes on one serial bus

system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

13.5.1 F/S-mode transfer in a mixed-speed bus

system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

13.5.2 Hs-mode transfer in a mixed-speed bus

system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

13.5.3 Timing requirements for the bridge in a

mixed-speed bus system. . . . . . . . . . . . . . 27

14 10-BIT ADDRESSING . . . . . . . . . . . . . . . . 27

14.1 Definition of bits in the first two bytes. . . . . 27

14.2 Formats with 10-bit addresses. . . . . . . . . . 27

14.3 General call address and start byte with

10-bit addressing. . . . . . . . . . . . . . . . . . . . 30

15 ELECT RI C AL SP ECIFI CAT IONS

AND TIMING FOR I/O STAGES

AND BUS LINES . . . . . . . . . . . . . . . . . . . . 30

15.1 Standard- and Fast-mode devices. . . . . . . 30

15.2 Hs-mode devices. . . . . . . . . . . . . . . . . . . . 34

16 ELECTRICAL CONNECTIONS OF

I2C-BUS DEVICES TO THE BUS LINES . 37

16.1 Maximum and minimum values of

re sist or s Rp and Rs for Standard-mode

I2C-bus devices . . . . . . . . . . . . . . . . . . . . . 39

17 APPLICATION INFORMATION. . . . . . . . . 41

17.1 Slope-controlled output stages of

Fast-mode I2C-bus devices . . . . . . . . . . . . 41

17.2 Switched pul l-up circuit for Fast- mode

I2C-bus devices . . . . . . . . . . . . . . . . . . . . . 41

17.3 Wiring pattern of the bus lines. . . . . . . . . . 42

17.4 Maximum and minimum values of

re sist or s Rp and Rs for Fast-mode

I2C-bus devices . . . . . . . . . . . . . . . . . . . . . 42

17.5 Maximum and minimum values of

re sist or s Rp and Rs for Hs-mode

I2C-bus devices . . . . . . . . . . . . . . . . . . . . . 42

18 BI-DIRECTIONAL LEVEL SHIFTER

FOR F/S-MODE I2C-BU S SYSTEMS . . . . 4 2

18.1 Connecting devices with different

logic levels. . . . . . . . . . . . . . . . . . . . . . . . . 43

18.1.1 Operation of the level shifter . . . . . . . . . . . 44

19 DEVE LO PMENT TOOLS AVAILABLE

FROM PHILIPS . . . . . . . . . . . . . . . . . . . . . 45

20 SUPPORT LITERATURE . . . . . . . . . . . . . 46

Philips Semiconductors

The I2C-bus specification

1 PREFACE

1.1 Version 1.0 - 1992

This ver sion of the 1 992 I2C-bus specifica tion inclu des the

following modifications:

•Programming of a slave address by software has been

omitted. The realization of this feature is rather

complicated and has not been used.

•The “low- speed mode” has be en omitted. This mode is,

in fact, a subset of the total I2C-bus specification and

need not be specified explicitly.

•The Fast-mode is added. This allows a fourfold increase

of the bit rate up to 400 kbit/s. Fast-mode devices are

downwards compatible i.e. they can be used in a 0 to

100 kbit/s I2C-bus system.

•10-bit addr es sing is adde d. Thi s al lo ws 1024 additional

slave addresses.

•Slope control and input filtering for Fast-mode devices is

specified to improve the EMC behaviour.

NOTE: Neither the 100 kbit/s I2C-bus system nor the

100 kbit/s devices have been changed.

1.2 Version 2.0 - 1998

The I2C-b us ha s bec ome a de fact o wor ld st andar d that i s

now implemented in over 1000 different ICs and licensed

to mor e than 50 c ompan ies. Many of tod ay’s appl icat ions,

however, require higher bus speeds and lower supply

voltages. This updated version o f the I2C-bus speci fication

meets those requirements and includes the following

modifications:

•The High- spee d mode (Hs-m ode) is adde d. Thi s allo ws

an increase in the bit rate up to 3.4 Mbit/s. Hs-mode

devices can be mixed with Fast- and Standard-mode

devices on the one I2C-bus s ystem wi th bit rates from 0

to 3.4 M bit/s.

•The low output level and hysteresis of devices with a

supply voltage of 2 V and below has been adapted to

meet the required noise margins and to remain

compatible with higher supply voltage devices.

•The 0.6 V at 6 mA requirement for the output stages of

Fast-mode devices has been omitted.

•The fixed input levels for new devices are replaced by

bus voltage-related levels.

•Application information for bi-directional level shifter is

added.

1.3 Version 2.1 - 2000

Version 2.1 of the I2C-bus specification includes the

following minor modifications:

•After a repeated START condition in Hs-mode, it is

possible to stretch the clock signal SCLH (see

Section 13.2 and Figs 22, 25 and 32).

•Some t iming parameters i n Hs-mode h ave been relaxed

(see Tables 6 and 7).

1.4 Purchase of Philips I2C-bus components

Purchase of Philips I2C components conveys a license under the Philips’ I2C patent to use the

compo nents i n the I2C syste m provided the sys tem confo rms to the I 2C spec ificati on defined by

Philips.

Philips Semiconductors

The I2C-bus specification

2THE I

2C-BUS BENEFITS DESIGNERS AND

MANUFACTURERS

In consumer electronics, telecommunications and

industrial electronics, there are often many similarities

between seemingly unrelated designs. For example,

nearly every system includes:

•Some intelligent control, usually a single-chip

microcontroller

•General-purpose circuits like LCD drivers, remote I/O

ports, RAM, EEPROM, or data converters

•Application-oriented circuits such as digital tuning and

signal processing circuits for radio and video systems, or

DTMF generators for telephones with tone dialling.

To explo it th es e sim il ar iti es to the benefi t of both syst ems

designers and equipment manufacturers, as well as to

maximize hardware efficiency and circuit simplicity, Philips

developed a simple bi-directional 2-wire bus for efficient

inter-IC control. This bus is called the Inter IC or I2C-bus.

At present, Philips’ IC range includes more than 150

CMO S and bipolar I2C-bus compatible types for

performing functions in all three of the previously

mentioned categories. All I2C-bus compatible devices

incorporate an on-chip interface which allows them to

communicate directly with ea ch other via the I2C-bus. This

design concept solves the many interfacing problems

encountered when designing digital control circuits.

Here are some of the features of the I2C-bus:

•Only two b us lines are requir ed; a se rial data line ( SDA)

and a serial clock line (SCL)

•Each device connected to the bus is software

addressable by a unique address and simple

master/slave relationships exist at all times; masters can

operate as master-transmitters or as master-receivers

•It’s a true multi-master bus including collision detection

and arbitrati on to prev ent data corrupti on if two or more

masters simultaneously initiate data transfer

•Serial, 8-bit oriented, bi-directional data transfers can be

made at up to 100 kbit/s in the Standard-mode, up to

400 kbit/s in the Fast-mode, or up to 3.4 Mbit/s in the

High-speed mode

•On-chip filtering rejects spikes on the bus data line to

preserve data integrity

•The number of ICs that can be connected to the same

bus is limited only by a maximum bus capacitance of

400 pF.

Figure 1 shows two exam ples of I2C-bus application s.

2.1 Designer benefits

I2C-bus compati bl e ICs allow a system desi gn to rapidl y

progress directly from a functional block diagram to a

prototype. Moreover, since they ‘clip’ directly onto the

I2C-bus without any additional external interfacing, they

allow a prototype system to be modified or upgraded

simply by ‘clipping’ or ‘unclipping’ ICs to or from the bus.

Here are some of the features of I2C-bus compatible ICs

which are particularly attractive to designers:

•Function al blocks on th e block diag ram corres pond with

the actual ICs; designs proceed rapidly from block

diagram to final schematic.

•No need to design bus interfaces because the I2C-bus

interface is already integrated on-chip.

•Integrated addressing and data-transfer protocol allow

systems to be completely software-defined

•The same IC types can often be used in many different

applications

•Design-time reduces as designers quickly become

familiar with the frequently used functional blocks

represente d by I2C-bus compatible ICs

•ICs can be added to or removed from a system without

affecting any other circuits on the bus

•Fault diagnosis and debugging are simple; malfunctio ns

can be immediately traced

•Software development time can be reduced by

assembling a library of reusable software modu les.

In addition to these advantages, the CMOS ICs in the

I2C-bus compa tible ran ge offer designer s specia l featur es

which are particularly attractive for portable equipment and

battery-backed systems.

They all have:

•Extremely l ow current consum ption

•High noise immunity

•Wide supply voltage range

•Wide operating temperature range.

Philips Semiconductors

The I2C-bus specification

Fig.1 Two examples of I2C-bus applications: (a) a high performance highly-integrated TV set

(b) DECT cordless phone base-station.

handbook, full pagewidth

SDA SCL

MICRO-

CONTROLLER

PCB83C528

PLL

SYNTHESIZER

TSA5512

NON-VOLATILE

MEMORY

PCF8582E

STEREO / DUAL

SOUND

DECODER

TDA9840

HI-FI

AUDIO

PROCESSOR

TDA9860

SINGLE-CHIP

TEXT

SAA52XX

M/S COLOUR

DECODER

TDA9160A

PICTURE

SIGNAL

IMPROVEMENT

TDA4670

VIDEO

PROCESSOR

TDA4685

ON-SCREEN

DISPLAY

PCA8510

(a)

MSB575

SDA SCL

LINE

INTERFACE

PCA1070

BURST MODE

CONTROLLER

PCD5042

ADPCM

PCD5032

(b)

DTMF

GENERATOR

PCD3311

MICRO-

CONTROLLER

P80CLXXX

Philips Semiconductors

The I2C-bus specification

2.2 Manufacturer benefits

I2C-bus compatible ICs don’t only assist designers, they

also give a wide range of benefits to equipment

manufacturers because:

•The si mple 2-wire se rial I2C-bus minimizes

interconnections so ICs have fewer pins and there are

not so many PCB tracks; result - sm aller and less

expensive PCBs

•The completely integrated I2C-bu s protocol eliminates

the need for address decoders and other ‘glue logic’

•The multi-master capability of the I2C-bus allows rapid

testing and alignment of end-user equipment via

external connections to an assembly-line

•The availability of I2C-bus compatible ICs in SO (small

outline), VSO (very small outl ine) as we ll as DIL

packages reduces space requirements even more.

These are just some of the benefits. In addition, I2C-bus

compatible ICs increase system design flexibility by

allowing simple construction of equipment variants and

easy upgrading to keep desi gns up-to-date. In th is way, an

entire family of equipment can be developed around a

basic model. Upgrades for new equipment, or

enhanced-feature models (i.e. extended memory, remote

control, etc.) can then be produced simply by clipping the

appropriate ICs onto the bus. If a larger ROM is needed,

it’s simply a matter of sel ecting a micro-controller w ith a

larger ROM from our comprehensive range. As new ICs

supersede older ones, it’s easy to add new features to

equipment or to increase its performance by simply

uncli pping the outda ted IC from the bus and cl ipping on its

successor.

3 INTR ODUC TION TO THE I2C-BUS SPECIFICATION

For 8-bit oriented digital control applications, such as those

requiring microcontrollers, certain design criteria can be

established:

•A complete system usually consists of at least one

microcontroller and other peripheral devices such as

memories and I/O expanders

•The cost of connecting the various devices within the

system m ust be minimized

•A system that performs a control function doesn’t

require high-speed data transfer

•Overall efficiency depends on the devices chosen and

the nature of the interconnecting bus structure.

To produce a system to satisfy these criteria, a serial bus

structure is needed. Although serial buses don’t have the

throughput capability of parallel buses, they do require

less wi r ing and fewer I C co nnecting pins. H owev er , a bus

is not merely an interconnecting wire, it embodies all the

formats and procedures for communication within the

system.

Devices co mmunic ating wit h each o ther on a serial bu s

must have some form of protocol which avoids all

possibilities of confusion, data loss and blockage of

information. Fast devices must be able to communicate

with slo w dev i ce s. Th e s yst em mu st not be dependent o n

the devices connected to it, otherwise modifications or

improv ements would be im possible. A proc edure has also

to be devised to decide which device will be in control of

the bus and when. And, if different devices with different

clock speeds are connected to the bus, the bus clock

source must be defined. All these criteria are involved in

the specification of the I2C-bus.

4THE I

2C-BUS CONCEPT

The I2C-bus supports any IC fabrication process (NMOS,

CMOS, bipolar). Two w ire s, serial data (SDA) and serial

clock (SCL), carry information between the devices

connected to the bus. Each device is recognized by a

unique a ddress (whethe r i t’s a microcontroll er, LCD driver ,

memory or keyboard interface) and can operate as either

a transmitte r or recei ver, depen ding on the func tion of the

device. Obviously an LCD driver is only a receiver,

whereas a me mory c an both rec eive and tr ansmit data. In

addition to transmitters and receivers, devi ces can also be

considered as masters or slaves when performing data

transfers (see Table 1). A master is the device which

initiates a d ata transfer on the bus and ge nerates the clock

signals to permit that transfer. At that time, any device

addressed is considered a slave.

Philips Semiconductors

The I2C-bus specification

Table 1 Definition of I2C -bu s terminol ogy

The I2C-bus is a multi-master bus. This me ans that more

than one device capable of controlling the bus can be

connecte d to it. As masters are usually micro-controllers,

let’s consider the case of a data transfer between two

microcontrollers connected to the I2C-bus (see Fig.2).

This high li gh t s the master-slave and re ceiver-transmitter

relationships to be found on the I2C-bus. It should be noted

that these relationships are not permanent, but only

depend on the direction of data transfer at that time. The

transfer of data would proceed as follows:

1) Suppo se micro controller A wants to se nd inform ation to

microcontroller B:

•micr ocontro ller A ( master ), addre sses mi crocontr oller B

(slave)

•microcontroller A (master-transmitter), sends data to

microcontroller B (slave- receive r)

•microcontroller A terminates the transfer

2) If microcontroller A wants to receive information from

microcontroller B:

•microcontroller A (master) addresses microcontroller B

(slave)

•microcontroller A (master- receiver) receives d ata from

microcontroller B (slave- transmitter)

•microcontroller A terminates the transfer.

Even in this case, the master (microcontroller A) generates

the timing and terminates the transfer.

The possibility of connecting more than one

microcontroller to the I2C-bus means that more than one

master could try to initiate a data transfer at the same time.

To avoid th e chaos tha t might ens ue from suc h an event -

an arbitration procedure has been developed. This

procedure relies on the wired-AND connection of all I2C

interfaces to the I2C-bus.

If two o r mor e ma sters try to put infor ma t io n onto the bu s,

the first to produce a ‘one’ when the other produces a

‘zero’ will lose the arbitration. The clock signals during

arbitration are a synchronized combination of the clocks

generated by the masters using the wired-AND connection

to the SCL line (for more detailed information concerning

arbitration see Section 8).

TERM DESCRIPTION

Transmitter The device which sends data to the

bus

Receiver The device which receives data from

the bus

Master The device which initiates a transfer,

generates clock signals and

term inates a transfer

Slave The device addressed by a master

Multi -master More than one mas ter can a ttempt to

contro l the bus at the same tim e

without corrupting the message

Arbitration Procedure to ensure that, if more

than one mas ter s imul t an eous ly tries

to control the bus , only one is allow ed

to do so and the winning message is

not corrupted

Synchronization Procedure to synchronize the clock

signals of two or more devices

Fig.2 Example of an I2C-bus configuration using two microcontrollers.

MBC645

SDA

SCL

MICRO -

CONTROLLER

STATIC

RAM OR

EEPROM

LCD

DRIVER

GATE

ARRAY ADC

MICRO -

CONTROLLER

Philips Semiconductors

The I2C-bus specification

Generation of clock signals on the I2C-bu s is always th e

responsibility of master devices; each master generates its

own clock signals when transferring data on the bus. Bus

clock s ignals from a mas ter ca n only be altered when th ey

are stretched by a slow-slave device holding-down the

clock line, or by another master when arbitration occurs.

5 GENERAL CHARACTERISTICS

Both SDA an d SCL are bi-dire ctional lin es, connected to a

positive supply voltage via a current-source or pull-up

resistor (see Fig.3). When the bus is free, both lines are

HIGH. The ou tput stage s of dev i ces connecte d to th e bus

must have an open-drain or open-collector to perform the

wired-AND function. Data on the I2C-bus ca n be

transferred at rates of up to 100 kbit/s in the

Standard-mode, up to 400 kbit/s in the Fast-mode, or up to

3.4 Mbit/s in the High-speed mode. The number of

interfa ces connected to the bus is solely dependent on the

bus capacitance limit of 400 pF. For information on

High-speed mode master devices, see Section 13.

6 BIT TRANSFER

Due to th e var iety of d iffer ent tec hnolo gy devi ces ( CMOS,

NMOS, bipolar) which can be connected to the I2C-bus,

the levels of the logical ‘0’ (LOW) and ‘1’ (HIGH) are not

fixed and depend on the associated level of VDD (see

Section 15 for electrical specifications). One clock pulse is

generated for each data bit transferred.

6.1 Data validity

The data on the SDA line must be stable during the HIGH

period of the clock. The HIGH or LOW state of the data line

can only change when the clock signal on the SCL line is

LOW (see Fig.4).

Fig.3 Connection of Standard- and Fast-mode devices to the I2C-bus.

MBC631

SCLKN1

OUT

SCLK

DATAN1

OUT

DATA

DEVICE 1

SDA (Serial Data Line)

SCL (Serial Clock Line)

SCLKN2

OUT

SCLK

DATAN2

OUT

DATA

DEVICE 2

VDD

pull-up

resistors

Philips Semiconductors

The I2C-bus specification

Fig.4 Bit transfer on the I2C-bus.

handbook, full pagewidth

MBC621

data line

stable;

data valid

change

of data

allowed

SDA

SCL

6.2 START and STOP conditions

Within the procedure of the I2C-bus, unique situations

arise wh ich are defined as START (S) and STOP (P)

conditions (see Fig.5).

A HIGH to LOW transition on the SDA line while SCL is

HIGH is one such unique case. This situation indicates a

START condit io n .

A LOW to HIGH transition on the SDA line while SCL is

HIGH defines a STOP condition.

START and STOP conditio ns are always generated by the

master. The bus is consider ed to be busy af ter the START

conditi on. The bus is c onsidered to be free again a cert ain

time after the STOP condition. This bus free situation is

specified in Section 15.

The bus stays busy if a repeated START (Sr) is generated

instead of a STOP condition. In this respect, the START

(S) and repeated START (Sr) cond iti ons are functionally

identical (see Fig. 10). For the remainder of this document,

therefore, the S symbol will be used as a generic term to

represent both the START and repeated START

conditions, unless Sr is particularly relevant.

Detection of START and STOP conditions by devices

connected to the bus is easy if they incorporate the

necessary interfacing hardware. However,

microcontrollers with no such interface have to sample the

SDA line at least twice per clock period to sense the

transition.

Fig.5 START and STOP conditions.

handbook, full pagewidth

MBC622

SDA

SCL P

STOP condition

SDA

SCL

START condition

Philips Semiconductors

The I2C-bus specification

7 TRANSFERRING DATA

7.1 Byte format

Every byte put on the SDA line must be 8-bits long. The

number of bytes that can be transmitted per transfer is

unrestricted. Each byte has to be followed by an

acknowledge bit. Data is transferred with the most

significant bit (MSB) first (see Fig.6). If a slave can’t

receive or transmit another complete byte of data until it

has p erforme d some o ther funct ion, for ex ample servici ng

an inter nal interr upt, it can hold the c lock line SCL LOW to

force the master into a wait state. Data transfer then

continu es when the s lave is ready for an other byt e of data

and releases clock line SCL.

In some cases, it’s permitted to use a different format from

the I2C-bus forma t (for CBUS compatible d evices for

example). A message which starts with such an address

can be terminated by generation of a STOP condition,

even during the transmission of a byte. In this case, no

acknowledge is generated (see Section 10.1.3).

7.2 Acknowledge

Data transfer with acknowledge is obligatory. The

acknowledge-related clock pulse is generated by the

master. The tra n smitter releases the SDA line (HIGH)

during the acknowledge clock pulse.

The receiver must pull down the SDA line during the

acknowledge clock pulse so that it remains stable LOW

during the HIGH period of this clock pulse (see Fig.7). Of

course, set-up and ho ld times (specifie d in Section 15)

must also be taken into account.

Usually, a receiver which has been addressed is obliged to

generate an acknowledge after each byte has been

received, except when the message starts with a CBUS

address (see Section 10.1.3).

When a slav e doesn’ t acknowled ge the slav e address (for

example, it’s unable to receive or transmit because it’s

perf orming some real -time functi on), the data line must be

left HIGH by the slave. The master can then generate

either a STOP condition to abort the transfer, or a repeated

START condition to start a new transfer.

If a slave-receiver does acknowledge the slave address

but, some time later in the transfer cannot receive any

more data bytes, the maste r must again abort the transfer.

This is indicated by the slave generating the

not-acknowledge on the first byte to follow. The slave

leaves the data line HIGH and the master generates a

STOP or a repeated START condition.

If a master-receiver is involved in a transfer, it must signal

the end of data to the sla ve- tr ans mi tte r by no t ge ner ati ng

an acknowledge on the last byte that was clocked out of

the slav e. The slave- transmitter must relea se the data lin e

to allow the master to generate a STOP or repeated

START condition.

Fig.6 Data transfer on the I2C-bus.

handbook, full pagewidth

MSC608

SDA

SCL

STOP or

repeated START

condition

START or

repeated START

condition

1 2 3 - 8 9

ACK

7812

MSB acknowledgement

signal from slave

byte complete,

interrupt within slave

clock line held low while

interrupts are serviced

acknowledgement

signal from receiver

Philips Semiconductors

The I2C-bus specification

Fig.7 Acknowledge on the I2C-bus.

handbook, full pagewidth

MBC602

START

condition

9821

clock pulse for

acknowledgement

not acknowledge

acknowledge

DATA OUTPUT

BY TRANSMITTER

DATA OUTPUT

BY RECEIVER

SCL FROM

MASTER

8 ARBITRATION AND CLOCK GENERATION

8.1 Synchronization

All masters generate their own clock on the SCL line to

transfer messages on the I2C-bus. Data is only valid during

the HIGH period of the clock. A defined clock is therefore

needed for the bit-by-bit arbitration procedure to take

place.

Clock synchronization is performed using the wired-AND

connection of I2C interfaces to the SCL line. This means

that a HIGH to LOW transition on the SCL line will cause

the devices concerned to start counting off their LOW

period and, once a device cloc k has gone LOW, i t will hold

the SCL line in that state until the clock HIGH state is

reached (see Fig.8). However, the LOW to HIGH transition

of this clock may not change the state of the SCL line if

another clock is still within its LOW period. The SCL line

will therefore be held LOW by the device with the longest

LOW period. Devices with shorter LOW periods enter a

HIGH wait-state during this time.

Fig.8 Clock synchronization during the arbitration procedure.

CLK

SCL

counter

reset

wait

state

start counting

HIGH period

MBC632

Philips Semiconductors

The I2C-bus specification

When all devices concerned have counted off their LOW

period , the cloc k li ne will be r elea sed a nd go HIGH. Ther e

will then be no difference between the device clocks and

the state of the SCL line, and all the devices will start

counting their HIGH periods. The first device to complete

its HIGH period will again pull the SCL line LOW.

In this wa y, a synchronized S CL clock is generated wit h its

LOW period determined by the device with the longest

clock L OW perio d, a nd its HIGH pe ri od dete rmi ned by the

one with the shortest clock H IGH period.

8.2 Arbitration

A mast er may sta rt a transfe r only if the bus is fre e. Two or

more mast ers may generate a START condition w ithin the

minimum hold time (tHD;STA) of the START condition which

results in a defined START condition to the bus.

Arbit ration take s place on the SDA line, whi le the SCL line

is at the HIGH level, in such a way that the master which

transmits a HIGH level, while another master is

transmitting a LOW level will switch off its DATA output

stage bec ause the l ev el on the bus doesn’ t c or resp ond to

its own level.

Arbitration can continue for many bits. Its first stage is

comparison of the addr es s bi ts ( add ressi ng i nformati on is

given in Sections 10 and 14). If the masters are each trying

to address the same device, arbitration continues with

compar ison of the da ta-bi ts i f they ar e maste r-tr ansm itter ,

or acknowledge-bits if they are master-receiver. Because

address and data information on the I2C-bus is determined

by the winning master, no information is lost during the

arbitr ati on pr oc es s.

A master that loses the arbitration can generate clock

pulses until the end of the byte in which it loses the

arbitration.

As an Hs-mode master has a unique 8-bit master code, it

will always finish the arbitration during the first byte (see

Sectio n 13).

If a master also incorporates a slave function and it loses

arbitration during the addressing stage, it’s possible that

the winning master is trying to address it. The losing

mast er must ther efore switc h over immedi ately to it s slave

mode.

Figure 9 shows th e ar bi tr ati on procedure for tw o m as t er s.

Of course, more may be involved (depending on how

many masters are connected to the bus). The moment

there i s a d iffe r enc e b etwe en the internal data lev el of the

master generating DATA 1 and the actual level on the SDA

line, its data output is switched off, which means that a

HIGH output level is then connected to the bus. This will

not affect the dat a tra nsfer initia ted by the wi nning ma st er.

Fig.9 Arbitration p rocedur e of two master s.

handbook, full pagewidth

MSC609

DATA

SDA

SCL

master 1 loses arbitration

DATA 1 SDA

Philips Semiconductors

The I2C-bus specification

Since control of the I2C-bus is decided solely on the

address or master code and data sent by competing

masters, there is no central master, nor any order of

priority on the bus.

Special attention must be paid if, during a serial transfer,

the arbit ra tion p roce dure is s til l in prog ress at the m oment

when a repeate d START condi tion or a STOP condition is

transmitted to the I2C-bus. If it’s possible for such a

situation to occur, the masters involved must send this

repeated START condition or STOP cond ition at the same

position in the format frame. In other words, arbitration isn’t

allowed between:

•A repeated START condition and a data bit

•A STOP condition and a data bit

•A repeated START condition and a STOP condition.

Slaves are not involved in the arbitration procedure.

8.3 Use of the clock synchronizing mechanism as

a handshake

In addition to bei ng used during the ar bitratio n proc edure ,

the clock synchronization mechanism can be used to

enable rec eivers to cope wi th fast data transf ers, on either

a byte level or a bit level.

On the byte l evel, a devic e may be able to receive bytes of

data at a fast rate, but needs more time to store a received

byte or prepare anot her byt e to be tra nsmitted. Slav es can

then hold the SCL line LOW after reception and

acknowledgment of a byte to force the master into a wait

state unti l the sl av e is r eady for the nex t by te tr ansfer i n a

type of handshake procedure (see Fig.6).

On the bit level, a device such as a microcontroller with or

without limited hardware for the I2C-bus, can slow down

the bus clock by extending each clock LOW period. The

speed of any master is thereby adapted to the internal

operating rate of this device.

In Hs-mode, this handshake feature can only be used on

byte leve l (see Section 13).

9 FORMATS WITH 7-BIT ADDRESSES

Data transf ers follow the format sho wn in Fig. 10. After the

START co ndition (S), a slave address is sent. This

address is 7 bits long followed by an eighth bit which is a

data direct i on bi t (R/ W) - a ‘zero’ indicates a transmission

(WRITE), a ‘one’ indicates a request for data (READ). A

data transfer is always terminated by a STOP condition (P)

generate d by the m aster. Ho wever, if a master s till wi shes

to communicate on the bus, it can generate a repeated

START condition (Sr) and address another slave without

first generating a STOP condition. Various combinations of

read/wr ite formats ar e then possibl e within such a tr ansfer.

Fig.10 A complete data transfer.

handbook, full pagewidth

1 – 7 8 9 1 – 7 8 9 1 – 7 8 9

STOP

condition

START

condition DATA ACKDATA ACKADDRESS ACKR/W

SDA

SCL

MBC604

Philips Semiconductors

The I2C-bus specification

Possible data transfer formats are:

•Master-transmitter transmits to slave-recei ve r. The

transfer direction is not changed (see Fig.11).

•Master reads sla v e immediately after fir st byte (see

Fig.12). At the moment of the first acknowledge, the

master- transmitter becomes a master- receiver and the

slave-re ce iver becomes a slave-transmitter. This first

acknowledge is still generated by the slave. The STOP

condition is generated by the master, which has

previously sent a not-acknowledge (A).

•Combined format (see Fig.13). During a change of

directi on wi thin a trans fer, t he STA RT con ditio n and the

slave address are both repeated, but with the R/W bit

revers ed. If a mas ter recei ver se nds a repeated START

condition, it has previously sent a not-acknowledge (A).

NOTES:

1. Combined formats can be used, for example, to

contr ol a serial memory. Du ring the first data byte, the

internal memory location has to be written. After the

START co ndi tion an d slave a ddres s is r epeated , data

can be transferred.

2. All decisions on auto-increment or decrement of

previously accessed memory locations etc. are taken

by the designer of the device.

3. Each byte is followed by an acknowledgment bit as

indicated by the A or A blocks in the sequence.

4. I2C-bus compatible devices must reset their bus logic

on receipt of a START or repeated START condition

such that they all anticipate the sending of a slave

address, even if these START conditions are not

positioned according to the proper format.

5. A START condition immediately followed by a STOP

condition (v oid message) is a n illegal format .

Fig.11 A master-transmitter addressing a slave receiver with a 7-bit address.

The transfer direction is not changed.

handbook, full pagewidth

,,,

,,,,,,

MBC605

A/A

'0' (write) data transferred

(n bytes + acknowledge)

A = acknowledge (SDA LOW)

A = not acknowledge (SDA HIGH)

S = START condition

P = STOP condition

R/W

from master to slave

from slave to master

DATADATAASLAVE ADDRESSS P

Fig.12 A master reads a slave immediately after the first byte.

handbook, full pagewidth

,,,,,,

MBC606

(read) data transferred

(n bytes + acknowledge)

R/W A

PDATADATASLAVE ADDRESSS A

Philips Semiconductors

The I2C-bus specification

Fig.13 Combined format.

handbook, full pagewidth

,,,,,

MBC607

DATAAR/W

read or write

A/A

DATAAR/W

(n bytes

+ ack.)

direction

of transfer

may change

at this point.

read or write

(n bytes

+ ack.)

Sr = repeated START condition

A/A

*not shaded because

transfer direction of

data and acknowledge bits

depends on R/W bits.

SLAVE ADDRESSSSrPSLAVE ADDRESS

10 7-BIT ADDRESSING

The addres sing proc edur e for the I2C-bus is such that the

first byte after the START condition usually determines

which s la ve wi ll be s el ec ted by th e m as ter . The ex ce ption

is the ‘ge neral call’ a ddress which can address all de vices.

When this address is used, all devices should, in theory,

respond with an acknowledge. However, devices can be

made to ignore this address. The second byte of the

general call address then defines the action to be taken.

This procedure is explained in more detail in

Section 10.1.1. For information on 10-bit addressing, see

Section 14

10.1 Definition of bits in the first byte

The first seven bits of the first byte make up the slave

address (see Fig.14). The eighth bit is the LSB (least

signifi cant bit). It d etermines the directi on of the mes sage.

A ‘zero’ in the least significant position of the first byte

means that the master will write information to a selected

slave. A ‘one’ in this position means that the master will

read information from the slave.

When an address is sent, each device in a system

compares the first seven bits after the START condition

with its address. If they match, the device considers itself

addressed by the master as a slave-receiver or

slave-transmitter, depending on the R/W bit.

A slave address can be made-up of a fixed and a

programmable part. Since it’s likely that there will be

several identical devices in a system, the programmable

part of the slave address enables the maximum possible

number of such devices to be connected to the I2C-bus.

The number of programmable address bits of a device

depends on the number of pins available. For example, if

a device has 4 fixed and 3 programmable address bits, a

total of 8 identical devices can be connected to the same

bus.

The I2C-bus committee coordinates allocation of I2C

addresses. Further information can be obtained from the

Philips representatives listed on the back cover. Two

groups of eight addresses (0000XXX and 1111XXX) are

reserved for the purposes shown in Table 2. The bit

combination 11110XX of the slave address is reserved for

10-bit addressing (see Section 14).

Fig.14 The first byte after the START procedure.

handbook, halfpage

MBC608

R/W

LSBMSB

slave address

Philips Semiconductors

The I2C-bus specification

Table 2 Definition of bits in the first byte

Notes

1. No de vice i s al lowed to acknow ledge at the rec eption

of the START byte.

2. The CBUS address has been reserved to enable the

inter-mixing of CBUS compatible and I2C-bus

comp atible devices in the sam e system. I2C-bus

compatible devices are not allowed to respond on

reception of this address.

3. The address reserved for a different bus format is

included to enable I2C and other protocols to be mixed.

Only I2C-bus compatible devices that can work with

such formats and protocols are allowed to respond to

this address.

10.1.1 GENERAL CALL ADDRESS

The general call address is for addressing every device

connected to the I2C-bus. However, if a device doesn’t

need any of the data supplied within the general call

structure, it can ignore this address by not issuing an

acknowledgment. If a device does require data from a

general ca ll addr ess, it wil l ackn owledg e this add re ss and

behave as a slave- receiver. The second and following

bytes will be acknowledged by every slave- receiver

capable of handling this data. A slave which cannot

process one of these bytes must ignore it by

not-acknowledging. The meaning of the general call

address is always specified in the second byte (see

Fig.15).

There are two cases to consider:

•When the least significant bit B is a ‘zero’.

•When the least significant bit B is a ‘one’.

When bit B is a ‘zero’; the second byte has the following

definition:

•00000110 (H‘06’). Reset and write programmable part

of slave address by hardware. On receiving this 2-byte

sequence, all devices designed to respond to the

general call address will reset and take in the

programmable part of their address. Pre-cautions have

to be taken to ensure that a device is not pulling down

the SDA or SCL line after applying the supply voltage,

since these low levels would block the bus.

•00000100 (H‘04’). Write programmable part of slave

address by hardware. All devices which define the

programmable part of their address by hardware (and

which r espond to the gener al call addre ss) will latc h this

programmable part at the reception of this two byte

sequence. The device will not reset.

•00000000 (H‘00’). This code is not allowed to be used as

the second byte.

Sequences of programming procedure are published in

the appropriate device data sheets.

The remaining codes have not been fixed and devices

must ignore them.

When bit B is a ‘one’; the 2-byte sequence is a ‘hardware

general call’. This means that the se quence is tran smitted

by a hardware master device, such as a keyboard

scanner, which cannot be programmed to transmit a

desired slave address. Since a hardware master doesn’t

know in advance to which device the message has to be

transfe rred, it can only gene rate this hardware general call

and its own address - identifying itself to the system (see

Fig.16).

The seven bits remaining in the second byte contain the

address of the hardware master. This address is

recognized by an intelligent device (e.g. a microcontroller)

connect ed to the bus whic h will then dire ct the info rmation

from the hardware master. If the hardware master can also

act as a slave , th e sla ve ad dress is i dentic al to the ma ste r

address.

SLAVE

ADDRESS R/W BIT DESCRIPTION

0000 000 0 General call address

0000 000 1 START byte(1)

0000 001 X CBUS address(2)

0000 010 X Reser ved for different bus

format(3)

0000 011 X Reserved for future purposes

0000 1XX X Hs-mode master code

1111 1XX X Reserved for future purposes

1111 0XX X 10-bit slave addressing

Fig.15 General call address format.

MBC623

LSB

second byte

0 0 0 0 0 0 0 0 A X X X X X X X BA

first byte

(general call address)

Philips Semiconductors

The I2C-bus specification

Fig.16 Data transfer from a hardware master-transmitter.

handbook, full pagewidth

,,,

,,,,,

MBC624

general

call address

(B)

A A

second

byte

A A

(n bytes + ack.)

S 00000000 MASTER ADDRESS 1 PDATA DATA

In some systems, an alternative could be that the

hardwa re master transmitter is set in the slave-receiver

mode after the system reset. In this way, a system

configuring master can tell the hardware master-

transm itter (which is now in s lave-receive r mode) to which

address data must be sent (see Fig.17). After this

programming procedure, the hardware master remains in

the master-tra nsmitter mode.

10.1.2 START BYTE

Microc ontr o llers can be connec te d to the I2C-bus in two

ways. A micr oco ntrol ler wi th a n on-c hip ha rdwar e I2C-bus

interface can be programmed to be only interrupted by

requests from the bus. When the device doesn’t have such

an interface, it must constantly monitor the bus via

software. Obviously, the more times the microcontrol le r

monitors, or polls the bus, the less time it can spend

carrying out its intended function.

There is therefore a speed difference between fast

hardwa re devices and a relatively slow microco ntroller

which relies on software polling.

In this case, data transfer can be preceded by a start

proced ure which i s much longe r than nor mal (s ee Fig.18) .

The start procedure consists of:

•A START condition (S)

•A START byte (00000001)

•An ackno wledge cloc k pul se (ACK)

•A repeated START conditio n (Sr).

Fig.17 Da ta transfer by a ha rdware-t r an sm it t er c ap abl e of dum ping data dir ect ly to s l av e devices .

(a) Configuring master sends dump address to hardware master

(b) Hardware master dumps data to selected slave.

handbook, full pagewidth

,,,,,,,

,,,,,,

MBC609

write

(a)

(b)

R/W

write

(n bytes + ack.)

A/A

R/WS PSLAVE ADDR. H/W MASTER DUMP ADDR. FOR H/W MASTER X

S PDUMP ADDR. FROM H/W MASTER DATA DATA

Philips Semiconductors

The I2C-bus specification

Fig.18 START byte procedure.

MBC633

9821

ACK

dummy

acknowledge

(HIGH)

start byte 00000001

SDA

SCL

After the START condition S has been transmitted by a

master whi ch requires bus acces s, the START byte

(00000001) is transmitted. Another microcontroller can

therefor e s am pl e the SD A line at a l ow s am pli ng rat e until

one of the seven zeros in the START byte is detected.

After detection of this LOW level on the SDA line, the

microcontroll er can switc h to a higher sampling rate to find

the repeated START condition Sr which is then used for

synchronization.

A hardware receiver will reset on receipt of the repeated

START condition Sr and will ther efo r e ignor e the STA RT

byte.

An acknowledge-relat ed clock pulse is generat ed after the

START by te. This is presen t only to con form w ith the byte

handling format used on the bus. No device is allowed to

acknowledge the START byte.

10.1.3 CBUS COMPATIBILITY

CBUS receivers can be connected to the Standard-mode

I2C-bus . Howeve r, a thi rd bus line ca lled DL EN mu st th en

be connec ted and the ackno wledge bi t omitt ed. Norm ally ,

I2C transmissions are sequences of 8-bit bytes; CBUS

compatible devices have different formats.

In a mixed bus structure, I2C-bus devices must not

respon d to the CBUS messag e. For this reason, a speci al

CBUS address (0000001X) to which no I2C-bus

compatible device will respond, has been reserved. After

transm is s ion of the CBUS address, the DLEN line can be

made active and a CBUS-fo rmat transmission sen t (see

Fig.19). After the STOP condition, all devices are again

ready to acc ept data .

Master-tra nsmitters can send CBUS formats after send ing

the CBUS addres s. The transmi ssion is ended by a STOP

condition, recognized by all devices.

NOTE: If the CBUS confi guration is known, and expansion

with CBUS compatible devices isn’t foreseen, the designer

is allowed to adapt the hold time to the specific

require ments of the de vice(s) used .

Philips Semiconductors

The I2C-bus specification

MBC634

STOP

condition

CBUS

load pulse

n - data bitsCBUS

address

START

condition R/W

bit ACK

clock pulse

SDA

SCL

DLEN

Fig.19 Data format of transmissions with CBUS transmitter/receiver.

11 EXTEN SIONS T O THE STANDAR D-MODE I2C-BUS

SPECIFICATION

The Standard-mode I2C-bus specification, with its data

transfer rate of up to 100 kbit/s and 7-bit addressing, has

been in existence since the beginning of the 1980’s. This

concept rapidly grew in popularity and is today accepted

worldwide as a de facto standard with several hundred

different compatible ICs on offer from Philips

Semiconductors and othe r su ppliers. To meet the

demands for higher speeds, as well as make available

more slave address for the growing number of new

devices, the Standard-mode I2C-bus specification was

upgraded over the years and today is available with the

following extensions:

• Fast-mode, with a bit rate up to 400 kbit/s.

• High-speed mode (Hs-mode), with a bit rate up to

3.4 Mbit/s.

•10-bit addressing, which allows the use of up to 1024

additional slave addresses.

There are two main reasons for extending the regular

I2C-bus specification:

•Many of today’s applications need to transfer large

amounts of serial d ata and require bit r ates far in excess

of 100 kbit/s (Standard-mode), or even 400 kbit/s

(Fast-mode). As a result of continuing improvements in

semiconductor technologies, I2C-bus devices are now

available with bit rates of up to 3.4 Mbit/s (Hs-mode)

without any noticeable increases in the manufacturing

cost of the interface circu itry.

•As most of the 112 addresses available with the 7-bit

addressing scheme were soon allocated, it became

apparent that more address combinations were required

to prevent problems with the allocation of slave

addresses for new devices. This problem was resolved

with the new 10-bit addressing scheme, which allowed

about a tenfold increase in available addresses.

New sla ve devices w ith a Fast- or Hs-mod e I2C-bus

interface can have a 7- or a 10-bit slave address. If

possible, a 7-bit address is preferred as it is the cheapest

hardware solution and results in the shortest message

length. Devices with 7- and 10-bit addresses can be mixed

in the same I2C- bus sy s t em re gar dl es s of whether it is an

F/S- or Hs-mode sys tem. Both e xisting a nd future ma sters

can generate either 7- or 10-bit addresses.

12 FAST-MODE

With the Fast-mode I2C-bus specification, the protocol,

format, logic levels and maximum capacitive load for the

SDA and SC L lines quoted i n the Stan dard- mode I2C-bus

specification are unchanged. New devi ces with an I2C-bus

interfac e must meet at least the minim um requirem ents of

the Fast- o r Hs-mode specifica ti on (see Section 13).

Fast-mode devices can receive and transmit at up to

400 kbit/s. The minimum requirement is that they can

synchronize with a 400 kbit/s transfer; they can then

prolong the LOW period of the SCL signal to slow down the

transfer. Fast-mode devices are downward-compatible

and can communicate with Standard-mode devices in a

0 to 100 kbit/s I2C-bu s sy st em. As Standard- mod e

devices, however, are not upward c ompatible, they should

not be incorporated in a Fast-mode I2C-bus system as

they cannot follow the higher transfer rate and

unpredictable states would occur.

Philips Semiconductors

The I2C-bus specification

The Fast-m ode I2C-bus specification has the following

additional features compared with the Standard-mode:

•The maximum bit rate is increased to 400 kbit/s.

•Timin g o f the serial data (SDA) and s erial clock (SCL )

signals has been adapted. There is no need for

compatibility with other bus systems such as CBUS

because they cannot operate at the increased bit rate.

•The inputs of Fast-mode devices incorporate spike

suppression and a Schmitt trigger at the SDA and SCL

inputs.

•The output buffers of Fast-mode devices incorporate

slope control of the falling edges of the SDA and SCL

signals.

•If the power supply to a Fast-mode device is switched

off, the SDA and SCL I/O pins must be floating so that

they don’t obstruct the bus lines.

•The external pull- up devi ces connected to the bus line s

must be adapted to accommodate the shorter maximum

permissible rise time for the Fast-mode I2C-b us. For bus

loads up to 200 pF, the pull-up device for each bus line

can be a resistor; for bus loads between 200 pF and

400 pF, the pull-up device can be a current source

(3 mA max.) or a switched resistor circuit (see Fig.43).

13 Hs-MODE

High-speed mode (Hs-mode) devices offer a quantum

leap in I2C-bus transfer speeds. Hs-mode devices can

transf er information at bit rates of up to 3.4 Mbit/s, yet they

remain fully do wnward compatib le with F ast- or

Standard-mode (F/S-mode) devices for bi-directional

communi ca t ion in a mixed-speed bus system. With the

except ion that arbitrati on and cloc k synchr oniz ation is not

performed during the Hs-mode transfer, the same serial

bus protocol and data format is maintained as with the

F/S-mode system. Depending on the application, new

devices may have a Fast or Hs-mode I2C-bus interface,

although Hs-mode devices are preferred as they can be

designed-in to a greater number of applications.

13.1 High speed transfer

To achieve a bit transfer of up to 3.4 Mbit/s the following

improvements have been made to the regular I2C-bus

specification:

•Hs-mode master devices have an open-drain output

buffer for the SDAH signal and a combination of an

open-drain pull-down and current-source pull-up circuit

on the SCLH output(1). This current-so urce circuit

shortens the rise time of the SCLH signal. Only the

current-source of one master is enabled at any one time,

and only during Hs-mode.

•No arbitration or clock synchronization is performed

during Hs-mode transfer in multi-master systems, which

speeds-up bit handling capabilities. The arbitration

procedure always finishes after a preceding master

code transmission in F/S-mode.

•Hs-mode master devices generate a serial clock signal

with a HIGH to LOW ratio of 1 to 2. This relieves the

timing requirements for set-up and hold times.

•As an option, Hs-mode master devices can have a

built-in bridge(1). During Hs-mode transfer, the high

speed data (SDAH) and high-speed seri al clock (SCLH)

lines of Hs-mode devices are separated by this bridge

from the SDA and SCL line s of F/S-mode dev ice s. This

reduces the capacitive load of the SDAH and SCLH

lines resulting in faster rise and fall times.

•The only difference between Hs-mode slave devices

and F/S-mode slave devices is the speed at which they

operate. Hs-mode slaves have open-drain output buffers

on the SCLH an d SDAH ou tputs. Optional pull-d own

transistors on the SCLH pin can be used to stretch the

LOW level of the SCLH signal, although this is only

allowed after the acknowled ge bit in Hs-mode transfers.

•The inputs of Hs-mode devices incorporate spike

suppression and a Schmitt trigger at the SDAH and

SCLH inputs.

•The output buffers of Hs-mode devices incorporate

slope control of the falling edges of the SDAH and SCLH

signals.

Figure 20 shows the physical I2C-bus configuration in a

system with only Hs- mode devic es. Pins SD A and SCL on

the master devices are only used in mixed-speed bus

systems and are not connected in an Hs-mode only

system. In such cases, these pins can be used for other

functions.

Optional se ries resistors Rs protect the I/O stages of the

I2C-bus devices from hi gh-v oltag e spik es on the bus lines

and minimize ringing and interference.

Pull-up resistors Rp main tain the SDAH and SCLH lin es at

a HIGH level when the bus is free and ensure the signals

are pulled up from a LOW to a HIGH level within the

required rise time. For higher capacitive bus-line loads

(>100 p F ), the resistor Rp can be replaced by external

current source pull-ups to meet the rise time requirements.

Unless pr oceeded by an ac kn owledge bi t, the ris e ti me of

the SCLH c lock puls es in Hs -m ode transfer s i s s ho rt ened

by the inte rnal current-source pull -up circuit MC S of the

active master.

(1) Patent application pending.

Philips Semiconductors

The I2C-bus specification

Fig.20 I2C-bus configuration with Hs-mode devices only.

handbook, full pagewidth

MSC612

VSS

SLAVE

SDAH SCLH

VSS

MASTER/SLAVE

SDAH SCLH SDA

MCS

SCL

RsRs

SLAVE

SDAH SCLH

VSS

RsRsRsRs

VDD

VSS

MASTER/SLAVE

SDAH SCLH SDA SCL

RsRs

VDD

(1) (1)(1) (1)

(2) (2)

(4) (4) (3) MCS(3)

(2) (2) (2) (2) (2) (2)

VDD

SCLH

SDAH

(1) SDA and SCL are not used here but may be use d for othe r functions.

(2) To input filter.

(3) Only the active master can enable its current-source pull-up circuit

(4) Dotted tr ansistors are optional open-drain out put s whic h can stretch the serial clock signal SCLH.

13.2 Serial data transfer format in Hs-mode

Serial data transfer format in Hs-mode meets the

Standard-mode I2C-bus specification. Hs-mode can only

commence after the following conditions (all of which are

in F/S-mode):

1. START condition (S)

2. 8-bit maste r code (00001X X X)

3. not-acknowledge bit (A)

Figures 21 and 22 show this in more detail. This master

code has two main functions:

•It allows arbitration and synchronization between

competing masters at F/S-mode speeds, resulting in

one winning mas ter .

•It indicates the beginning of an Hs-mode transfer.

Hs-mode master codes are reserved 8-bit codes, which

are not used for slave addressing or other purposes.

Furthermore, as each master has its own unique master

code, up to eigh t Hs- mode mas ters can be pres ent on th e

one I2C-bus system (although master code 0000 1000

should be reserv ed for tes t and diagno stic purpose s). The

mast er code for an Hs-mode master devic e is softwa re

programmable and is chosen by the System Designer.

Arbitration and clock synchronization only take place

during the transmission of the master code and

not-acknowledge bit (A), after which one winning master

remains active. The master code indicates to other devices

that an Hs-mode transfer is to begin and the connected

devices must meet the Hs-mode specification. As no

device is allowed to acknowledge the master code, the

master code is followed by a not-acknowledge (A).

After the not-acknowledge bit (A), and the SCLH line has

been pulled-up to a HIGH level, the active master switches

to Hs-mode and enables (at time tH, see Fig.22) the

current-source pull-up circuit for the SCLH signal. As other

devices can delay the serial transfer before tH by stretching

the LOW peri od of the SCL H signa l, th e activ e maste r wil l

enable its current-source pull-up circuit when all devices

have released the SCLH line and the SCLH signal has

reached a HIGH lev el, thus speedi ng up the last p art of the

rise time of the SCLH si gnal.

The active master then sends a repeated START condition

(Sr) followed by a 7-bit slave address (or 10-bit slave

Philips Semiconductors

The I2C-bus specification

address, see Section 14) with a R/W bit address, and

receives an acknowledge bit (A) from the selected slave.

After a repeated START condition and after each

acknowledge bit (A) or not-acknowledge bit (A), the active

master disables its current-source pull-up circuit. This

enables other devices to delay the serial transfer by

stretching the LOW period of the SCLH signa l. The active

master r e- enables its c ur r ent-source pull-up c ir c uit again

when all devices have released and the SCLH signal

reaches a HIGH level, and so speeds up the last part of

the SCLH signal’s rise time.

Data transfer continues in Hs-mode after the next

repeated START (Sr), and only switches back to

F/S-mode after a STOP condition (P). To reduce the

overhead of the master code, it’s possible that a master

links a number of Hs-mode transfers, separated by

repeated START conditions (Sr).

handbook, full pagewidth

,,,,,

F/S-mode Hs-mode (current-source for SCLH enabled) F/S-mode

MSC616

AA A/ADATA

(n bytes + ack.)

S R/WMASTER CODE Sr SLAVE ADD.

Hs-mode continues

,,,,

Sr SLAVE ADD.

Fig.21 Data transfer format in Hs-mode.

handbook, full pagewidth

MSC618

8-bit Master code 00001xxx AtH

F/S mode

Hs-mode If P then

F/S mode

If Sr (dotted lines)

then Hs-mode

16789 67891

1 2 to 5

2 to 5

6789

SDAH

SCLH

SDAH

SCLH

tHtFS

Sr Sr P

n × (8-bit DATA + A/A)

7-bit SLA R/W A

= MCS current source pull-up

= Rp resistor pull-up Fig.22 A complete Hs-mode transfer.

Philips Semiconductors

The I2C-bus specification

13.3 Switching from F/S- to Hs-mode and back

After reset and initialization, Hs-mode devices must be in

Fast-mode (which is in effect F/S-mode as Fast-mode is

downward compatible with Standard-mode). Each

Hs-mode device can switch from Fast- to Hs-mode and

back an d is control led by the seria l transfer o n the I2C-bus.

Before time t1 in Fig.22, each connected device operates

in Fast-mode. Between times t1 and tH (this time interval

can be stretched by any device) each connected device

must r ecognize th e “S 000 01XXX A” sequen ce and ha s to

switch it s in ternal circ uit fr om th e Fast- mode settin g to the

Hs-mod e setting. Between times t1 and tH the connected

master and slave devices perform this switching by the

following actions.

The active (winning) master:

1. Adapts its SDAH and SC LH input fil ter s accord ing to

the spike suppression requirement in Hs-mode.

2. Adapts the set-up and hold times according to the

Hs-mode requirements.

3. Adapts the slope control of its SDAH and SCLH output

stages according to the Hs-mode requirement.

4. Switches to the Hs-mode bit-rate, which is required

after time tH.

5. E nables the cu rrent source pul l-up circuit of its SCLH

output stage at time tH.

The non-active, or losing masters:

1. Adapt th eir SDAH a nd SCLH inpu t filter s ac cording to

the spike suppression requirement in Hs-mode.

2. Wait for a STOP condition to detect when the bus is

free again.

All slaves:

1. Adapt th eir SDAH a nd SCLH inpu t filter s ac cording to

the spike suppression requirement in Hs-mode.

2. Adapt the set-up and hold times according to the

Hs-mode requirements. This requirement may already

be fulfilled by the adaptation of the input filters.

3. Adapt the s lope contro l of their SDAH outp ut stages, i f

necessary. For slave devices, slope cont rol is

applicable for the SDAH output stage only and,

depending on circuit tolerances, both the Fast- and

Hs-mode requirements may be fulfilled without

switching its internal circuit.

At time tFS in Fig.22, each connected device must

recogni ze the STOP condition (P) and switch its internal

circuit from the Hs-mode setting back to the Fast-mode

setting as present before time t1. This must be completed

within the minimum bus free time as specified in Table 5

according to the Fast-mode specification.

Philips Semiconductors

The I2C-bus specification

13.4 Hs-mode devices at lower speed modes

Hs-mode devices are fully downwards compatible, and

can be connected to an F/S-mode I2C-bus s ystem (see

Fig.23). As no master code will be transmitted in such a

configuration, all Hs-mode master devices stay in

F/S-mode and communicate at F/S-mode speeds with

their current-source disable d. Th e SDAH and SCLH pins

are use d to connect to the F/ S-mode b us system, al lowing

the SD A and SCL pins (if present ) on the Hs -mode mast er

device to be used for other functions.

Fig.23 Hs-mode devices at F/S-mode speed.

handbook, full pagewidth

VSS VSS

Hs-mode

SLAVE

SDAH SCLH

VSS

Hs-mode

MASTER/SLAVE

SDAH SCLH SDA SCL

RsRs

Hs-mode

SLAVE

SDAH SCLH

VSS

RsRs

F/S-mode

MASTER/SLAVE

SDA SCL

RsRs

F/S-mode

SLAVE

SDA SCL

VSS

RsRs

VDD

(1)

(2) (2)

(4) (4) (4)

(2) (2) (2) (2) (2) (2) (2) (2)

(3)

(1)

VDD

SCL

SDA

MSC613

(1) Bridge not used. SDA and SCL may have an alternative function.

(2) To input filter.

(3) The current-source pull-up circuit stays disabled.

(4) Dotted transistors are optional open-drain outputs which can stretch the serial clock signal SCL.

13.5 Mixed speed modes on one serial bus system

If a system has a combination of Hs-, Fast- and/or

Standard-mode devices, it’s possible, by using an

intercon nectio n bridg e, to ha ve differen t bi t rat es bet ween

different devices (see Figs 24 and 25).

One bri dge is r equir ed to con nect/disco nnect an Hs-mode

section to/from an F/S-mode section at the appropriate

time. This bridge i nc lud es a level shi ft func ti on that allows

devices with different supply voltages to be connected. For

example F/S-mode devices with a VDD2 of 5 V can be

connected to Hs-mode devices with a VDD1 of 3 V or less

(i.e. where VDD2 ≥ VDD1), p ro vi ded SDA and SCL pins are

5 V tolerant. This bridge is incorporated in Hs-mode

master devices and is completely controlled by the serial

signals SDAH, SCLH, SDA and SCL. Such a bridge can be

implemented in any IC as an autonomous circuit.

TR1, TR2 and TR3 are N-channel transistors. TR1 and

TR2 have a transfer gate function, and TR3 is an open-

drain pull -down s tage. If TR1 or TR2 are s witc hed on they

transfer a LOW level in both directions, otherwise when

both the dr ain and so urce ris e to a HIGH leve l there wil l be

a high impedance between the drain and source of each

switched on transistor. In the latter case, the transistors will

act as a level shif ter as SDA H and SCLH wi ll be pull ed-up

to VDD1 and SDA and SCL will be pulled-up to VDD2

During F/S-mode speed, a bridge on one of the Hs-mode

masters connects the SDAH and SCLH lines to the

corresponding SDA and SCL lines thus permitting

Hs-mo de devices to co mmunica te with F/ S-mode devi ces

at slower speeds. Arbitration and synchronization is

possible during the total F/S-mode transfer between all

connected devices as described in Section 8. During

Hs-mode tr a nsfer, howev er , the bridge opens to separate

the two bus sections and allows Hs-mode devices to

communicate with each other at 3.4 Mbit/s. Arbitration

between Hs-mode devices and F/S-mode devices is only

performed during the master code (00001XXX), and

normally won by one Hs-mode master as no slave address

has four leading zeros. Other masters can win the

arbitration only if they send a reserved 8-bit code

(00000XXX). In such cases, the bridge remains closed and

the transfer proceeds in F/S-mode. Table 3 gives the

possible communication speeds in such a system.

Philips Semiconductors

The I2C-bus specification

Table 3 Communication bit-rates in a mixed speed bus system

TRANSFER

BETWEEN

SERIAL BUS SYSTEM CONFIGURATION

Hs + FAST +

STANDARD Hs + FAST Hs + STANDARD FAST +

STANDARD

Hs <–> Hs 0 to 3.4 Mbit/s 0 to 3.4 Mbit/s 0 to 3.4 Mbit/s –

Hs <–> Fast 0 to 100 kbit/s 0 to 400 kbit/s – –

Hs <–> Standard 0 to 100 kbit/s – 0 to 100 kbit/s –

Fast <–> Standard 0 to 100 kbit/s – – 0 to 100 kbit/s

Fast <–> Fast 0 to 100 kbit/s 0 to 400 kbit/s – 0 to 100 kbit/s

Standard <–> Standard 0 to 100 kbit/s – 0 to 100 kbit/s 0 to 100 kbit/s

MSC61

VSS

Hs-mode

SLAVE

SDAH SCLH

VSS

Hs-mode

MASTER/SLAVE

SDAH SCLH SDA SCL

RsRs

Hs-mode

SLAVE

SDAH SCLH

VSS

RsRs

F/S-mode

MASTER/SLAVE

SDA

SDAH

SCLH

SDA

SCL

VSS

RsRs

F/S-mode

SLAVE

SDA SCL

VSS

RsRs

VDD

VSS

Hs-mode

MASTER/SLAVE

VDD

VDD1

VDD2

SCLH

SDAH

MCSMCS (3)

(3)

(2)(2) (2)(2) (2)(2) (2)(2)(2)(2)(2)

(4) (4) (4)

(2)

(1) (1)

BRIDGE

TR1

TR3

TR2

Fig.24 Bus system with transfer at Hs- and F/S-mode speeds.

(1) Bridge not used. SDA and SCL may have an alternative function.

(2) To input filter.

(3) Only the active master can enable its current-source pull-up circuit.

(4) Dotted transistors are optional open-drain outputs which can stretch the serial clock signal SCL or SCLH.

13.5.1 F/S-MODE TRANSFER IN A MIXED-SPEED BUS

SYSTEM

The bridge shown in Fig.24 interconnects corresponding

serial bus lines, form ing one serial bus system. As no

master code (00001XXX) is transmitted, the

curr ent-sou rce pull -up cir cuits s tay disa bled and all outp ut

stages are open-drain. All devices, including Hs-mode

devices, communicate with each other according the

protocol, format and speed of the F/S-mode I2C-bus

specification.

13.5.2 HS-MODE TRANSFER IN A MIXED-SPEED BUS

SYSTEM

Figure 25 shows the timing diagram of a complete

Hs-mode t ransfer, whi ch i s invoked by a START condi tion,

a master code, and a not-acknowledge A (at F/S-mode

speed) . Al though this timing di agram is sp lit in two parts, it

should be viewed as one timing diagram were time point tH

is a common point for both parts.

Philips Semiconductors

The I2C-bus specification

handbook, full pagewidth

MSC611

8-bit Master code 00001xxx AtH

F/S mode

Hs-mode If P then

F/S mode

If Sr (dotted lines)

then Hs-mode

16789

16789 67891

1 2 to 5

2 to 5

6789

SDAH

SCLH

SDA

SCL

SDAH

SCLH

SDA

SCL

tHtFS

Sr Sr P

n × (8-bit DATA + A/A)

7-bit SLA R/W A

= MCS current source pull-up

= Rp resistor pull-up

Fig.25 A complete Hs-mode transfer in a mixed-speed bus system.

The master code is r ec ogni z ed by the brid ge i n the activ e

or non-active master (see Fig.24). The bridge performs the

following actions:

1. Between t1 and tH (see Fig.25), transistor TR1 opens

to separate the SDAH and SDA lines, after which

transis tor TR3 close s to pull- down the SDA l ine to V SS.

2. When both SCLH and SCL become HIGH (tH in

Fig.25), transistor TR2 opens to separate the SCLH

and SCL lines. TR2 must be opened before SCLH

goes LOW after Sr.

Hs-mode transfer starts after tH with a repeated START

condition (Sr). During Hs-mode transfer, the SCL line stays

at a HIGH and the SDA line at a LOW steady-state level,

and so is prepared for the transfer of a STOP condition (P).

Philips Semiconductors

The I2C-bus specification

After each acknowledge (A) or not-acknowledge bit (A) the

active m aster disables its current-so urce pull-up circuit.

This enables other devices to delay the serial transfer by

stretchi ng the LOW pe r iod of the S CLH s ign al. The ac ti ve

master re-enables its current-source pull-up circuit again

when all devices are released and the SCLH signal

reaches a HIGH level, and so speeds up the last part of the

SCLH signal’s rise time. In irregular situations, F/S-mode

devices can close the bridge (TR1 and TR2 closed, TR3

open) a t any ti me by pull ing down th e SCL l ine for at le ast

1µs, e.g. to recover from a bus hang-up.

Hs-mode finishes with a STOP condition and brings the

bus system back into the F/S-mode. The active master

disabl es its current-source MCS when the STOP cond ition

(P) at SDAH is d etecte d ( tFS in Fig.25). The bridge also

recognizes this STOP condition and takes the following

actions:

1. Transistor TR2 closes after tFS to connect SCLH with

SCL; both of which are HIGH at this time. Transistor

TR3 opens a fter t FS, which r el eas es the SDA li ne and

allows it to be pulled HIGH by the pull-up resister Rp.

This is the STOP condition for the F/S-mode devices.

TR3 must open fast enough to ensure the bus free

time between the STOP condition and the earliest next

START condition is according to the Fast-mode

specification (see tBUF in Table 5).

2. When SDA reaches a HIGH (t2 in Fig.25) transistor

TR1 closes to connect SDAH with SDA. (Note:

interconnections are made when all lines are HIGH,

thus preventing spikes on the bus lines). TR1 and TR2

must be closed within the minimum bus free time

according to the Fast-mode specification (see tBUF in

Table 5).

13.5.3 TIMING REQUIREMENTS FOR THE BRIDGE IN A

MIXED-SPEED BUS SYSTEM

It can be seen from Fig.25 that the actions of the bridge at

t1, tH and tFS must be so fast that it does not affect the

SDAH and SCLH lines. Furthermore the bridge must meet

the related timing requirements of the Fast-mode

specification for the SDA and SCL lines.

14 10-BIT ADDRES SING

This section describes 10-bit addressing and can be

disregarded if only 7-bit addressing is used.

10-bit addressing is compatible with, and can be combined

with, 7-bit addressing. Using 10 bits for addressing

exploits the reserved combination 1111XXX for the first

seven bits of the first byte following a START (S) or

repeated START (Sr) condition as explained in Section

10.1. The 10-bit addressing does not affect the existing

7-bit addressing. Devices with 7-bit and 10-bit addresses

can be c onnected t o the sa me I 2C-bus, and both 7-bi t and

10-bit add res sing can be us ed in F/S- mode and Hs-mode

systems.

Although there are eight possible combinations of the

reserved address bits 1111XXX, only the four

combinations 11110XX are used for 10-bit addressing.

The remaining four combinations 11111XX are reserved

for future I2C-bus enhancements.

14.1 Definition of bits in the first two bytes

The 10-bi t slave ad dress is formed fr om the fi rst two byt es

following a START condition (S) or a repeated START

condit ion (Sr).

The first seven bits of the first byte are the combination

11110XX of which the last two bits (XX) are the two

most-significant bits (MSBs) of the 10-bit address; the

eighth bit of the first byte is the R/W bit that determines the

direction of the message. A ‘zero’ in the least significant

position of the first byte means that the master will write

information to a selected slave. A ‘one’ in this position

means tha t the master wi ll read in formation fr om the slave.

If the R/W bit is ‘zero’, then the second byte contains the

remain ing 8 bits (XX XXXXXX) of the 10-bi t address . If the

R/W bit is ‘one’, then the next byte contains data

transmitted from a slave to a master.

14.2 Formats with 10-bit addresses

Various combinations of read/write formats are possible

within a transfer that includes 10-bit addressing. Possible

data transfer formats are:

•Master-transmitter transmits to slave-receiver with a

10-bit slave address.

The transfer direction is not changed (see Fig.26). When

a 10-bit ad dress fol lows a START condi tion, each sl ave

compares the first seven bi ts of the first byte of the slave

addres s (11110XX) with it s own address and tes ts if the

eighth bit (R/W direction bit) is 0. It is possible that more

than one device will find a match and generate an

acknowledge (A1). All slaves that found a match will

compare the eight bits of the second byte of the slave

address (XXXXXXXX) with their own addre sses, but

only one slave will find a match and generate an

acknowledge (A2). The matching slave will remain

addressed by the master until it receives a STOP

Philips Semiconductors

The I2C-bus specification

condition (P) or a repeate d START con di t ion (Sr)

followed by a different slave address.

•Master-receiver reads slave- tran smitter with a 1 0-bit

slave address.

The transfer direction is changed after the second R/W

bit (Fig .27). Up to and including acknowledge bit A2, the

procedure is the same as that described for a

master-transmitter addressing a slave -receiver. After

the repeated START condition (Sr), a matching slave

reme mbers that it wa s addressed before. This slave

then checks if the first seven bits of the first byte of the

slave address following Sr are the same as they were

after the START condition (S), and tests if the eighth

(R/W) bit is 1. If there is a match, the slave considers that

it has been addressed as a transmitter and generates

acknowledge A3. The slave-transmitter remains

addressed un til it rec eives a STOP co nditio n (P) or until

it rece ives another re peated START conditio n (Sr)

followed by a different slave address. After a repeated

START condition (Sr), all the other slave devices will

also compare the first seven bits of the first byte of the

slave a ddres s (111 10XX) with their own a ddress es and

test the eighth ( R/W) bi t. Howeve r, none of them wi ll be

address ed be cause R/ W = 1 (for 10-bit devices), or the

11110XX slave address (for 7-bit devices) does not

match.

•Combined format. A master transmits data to a slave

and then reads data from the same slave (Fig.28). The

same mast er occup ies the bu s all the time. The trans fer

direction is changed after the second R/W bit.

•Combine d form at. A ma ster trans mits dat a to o ne sla ve

and then transmits data to another slave (Fig.29). The

same master occupies the bus all the time.

•Combine d format. 10-bit and 7-bit addressing combined

in one serial transfer (Fig.30). After each START

condition (S), or each repeated START condition (Sr), a

10-bit or 7-bit slave address can be transmitted.

Figure 30 shows h ow a master tra nsmits data t o a slave

with a 7-bit addr ess and then transmits data to a second

slav e with a 10-b it addr ess. The same master occup ies

the bus all the time.

NOTES:

1. Combined formats can be used, for example, to

contr ol a serial memory. Du ring the first data byte, the

internal memory location has to be written. After the

START co ndi tion an d slave a ddres s is r epeated , data

can be transferred.

2. All decisions on auto-increment or decrement of

previously accessed memory locations etc. are taken

by the designer of the device.

3. Each byte is followed by an acknowledgment bit as

indicated by the A or blocks in the sequence.

4. I2C-bus compatible devices must reset their bus logic

on receipt of a START or repeated START condition

such that they all anticipate the sending of a slave

address.

Fig.26 A master-transmitter addresses a slave-receiver with a 10-bit address.

handbook, full pagewidth

,,,,

,,,,,

MBC613

R/W A1

(write)

A2 AA/A

1 1 1 1 0 X X 0

SLAVE ADDRESS

1st 7 BITS

S DATA PDATA

SLAVE ADDRESS

2nd BYTE

Fig.27 A master-receiver addresses a slave-transmitter with a 10-bit address.

handbook, full pagewidth

,,,,,

,,,,

MBC614

R/W A1

(write)

A3 DATA DATAA2 R/W

(read)

1 1 1 1 0 X X 0 1 1 1 1 0 X X 1

AAPSr

SLAVE ADDRESS

1st 7 BITS SLAVE ADDRESS

2nd BYTE SLAVE ADDRESS

1st 7 BITS

Philips Semiconductors

The I2C-bus specification

Fig.28 Combined format. A master addresses a slave with a 10-bit address,

then transmits data to this slave and reads data from this slave.

handbook, full pagewidth

,,,,,

MBC615

(write)

ADATA DATAR/W

(read)

,,,

,,,,,

R/W A A AA/A

1 1 1 1 0 X X

1 1 1 1 0 X X 1

SSLAVE ADDRESS

1st 7 BITS SLAVE ADDRESS

2nd BYTE DATA DATA

APASr SLAVE ADDRESS

1st 7 BITS

handbook, full pagewidth

,,,,

,,,,,

MBC616

(write)

AAA/A

(write)

,,,,

,,,,,

A A AA/A

1 1 1 1 0 X X

SSLAVE ADDRESS

1st 7 BITS R/W SLAVE ADDRESS

2nd BYTE DATA DATA

Sr SLAVE ADDRESS

1st 7 BITS R/W 2nd BYTE

SLAVE ADDRESS DATA PDATA

Fig.29 Combined format. A master transmits data to two slaves, both with 10-bit addresses.

Fig.30 Combined format. A maste r transmits data to two slaves, one wi th a 7-bit address,

and one with a 10-bit address.

book, full pagewidth

,,,,

,,,,,

,,,,

MBC617

R/W A

(write)

A A A/AR/W

(write)

AA/A

1 1 1 1 0 X X

SSLAVE ADDRESS

7 - BIT DATADATA

DATADATA P

SLAVE ADDRESS

1st 7 BITS OF 10-BIT

Sr SLAVE ADDRESS

2nd BYTE OF 10-BIT

Philips Semiconductors

The I2C-bus specification

14.3 General call address and start byte with 10-bit

addressing

The 10-bit addressing procedure for the I2C-bus is such

that the first two bytes after the START condition (S)

usually determine which slave will be selected by the

master. The exception is the “general call” address

00000000 (H‘00’). Slave devices with 10-bit addressing

will react to a “general call” in the same way as slave

devices with 7-bit addressing (see Section 10.1.1).

Hardware maste rs can tra nsmit their 10-bit address after a

‘general call’. In this case, the ‘general cal l’ address byte is

followed by two successive bytes containing the 10-bit

address of the master-transmitter. The format is as shown

in Fig.10 where the first DATA byte contains the eight

least-significant bits of the master address.

The START byte 000 00001 (H‘01’) can precede the 10- bit

addressing in the same way as for 7-bit addressing (see

Section 10.1.2).

15 ELECTRICAL SPECIFICATIONS AND TIMING FOR

I/O STAGES AND BUS LINES

15.1 Standard- and Fast-mode devices

The I/O levels, I/O current, spike suppression, output slope

control and pin capacitanc e for F/S-mode I 2C-bus devices

are given in Table 4. The I2C-bus timing characteristics,

bus-line capacitance and noise margin are given in

Table 5. Figure 31 shows the timing definitions for the

I2C-bus.

The minimum HIGH and LOW periods of the SCL clock

specified in Table 5 determine the maximum bit transfer

rates of 100 kbit/s for Standard-mode devices and

400 kbit/s for Fast-mode devices. Standard-mode and

Fast-mode I2C-bus devices must be able to follow

transfers at their own maximum bit rates, either by being

able to trans mit or receive at that spee d or by apply ing the

clock synchronization procedure described in Section 8

which wi ll force t he master i nto a wait state and stre tch the

LOW period of the SCL signal. Of course, in the l atter case

the bit transfer rate is reduced.

Philips Semiconductors

The I2C-bus specification

Table 4 Characteristics of the SDA and SCL I/O stages for F/S-mode I2C-bus devices

Notes

1. Devices that use non-standard supply voltages which do not conform to the intended I2C-bus system levels must

relate their input levels to the VDD voltage to which the pull-up resistors Rp are connected.

2. Maximum VIH = VDDmax + 0.5 V.

3. Cb = capacitance of one bus line in pF.

4. The maximum tf for the SDA and S CL bus lin es quo ted in Table 5 (300 ns) is longer than the s pecif ied maxi mum tof

for the output stages (250 ns). This allows series protection resistors (Rs) to be connected between the SDA/SCL

pins and the SDA/SCL bus lines as shown in Fig.36 without exceeding the maximum specified tf.

5. I/O pins of Fast-mode devices must not obstruct the SDA and SCL lines if VDD is switched off.

n/a = not applicable

PARAMETER SYMBOL STANDARD-MODE FAST-MODE UNIT

MIN. MAX. MIN. MAX.

LOW level input voltage:

fixed input levels

VDD-related input levels

VIL −0.5

−0.5 1.5

0.3VDD

n/a

−0.5 n/a

0.3VDD (1) V

HIGH level input voltage:

fixed input levels

VDD-related input levels

VIH 3.0

0.7VDD

(2)

(2) n/a

0.7VDD(1) n/a

(2) V

Hysteresis of Schmitt trigger inputs:

VDD > 2 V

VDD < 2 V

Vhys n/a

n/a n/a

n/a 0.05VDD

0.1VDD

–

–V

LOW level output voltage (open drain or

open collector) at 3 mA sink current:

VDD > 2 V

VDD < 2 V VOL1

VOL3

n/a 0.4

n/a 0

00.4

0.2VDD

Output fall time from VIHmin to VILmax with

a bus capacitance from 10 pF to 400 pF tof – 250(4) 20 + 0.1Cb(3) 250(4) ns

Pulse width of spikes which must be

suppressed by the input filter tSP n/a n/a 0 50 ns

Input current each I/O pin with an input

v oltage between 0.1VDD and 0.9VDDmax

Ii−10 10 −10(5) 10(5) µA

Capacitance for each I/O pin Ci−10 −10 pF

Philips Semiconductors

The I2C-bus specification

Table 5 Characteristics of the SDA and SCL bus lines for F/S-mode I2C-bus devices(1)

Notes

1. A ll values referr ed to VIHmin and VILmax levels (see Table 4).

2. A device must internally provide a hold time of at least 300 ns for the SDA signal (referred to the VIHmin of the SCL

signal) to bridge the undefined region of the falling edge of SCL.

3. The maximum tHD;DAT has only to be met if the device does not stretch the LOW period (tLOW) of the SCL signal.

4. A Fast-mo de I2C -bus dev ice c an be u sed in a Stan dard-m ode I2C-bus sys tem, but the r equirement tSU;DAT ≥250 ns

must then be met. This will automatically be the case i f the device does not stret ch the LOW per iod of the SCL signal.

If such a device does s tretch the LOW peri od of the SCL s ignal , it must out put the n ext da ta bi t to the SDA l ine tr max

+ tSU;DAT = 1000 + 250 = 1250 ns (according to the Standard-mode I2C-bus specification) before the SCL line is

released.

5. Cb = t otal capacitance of one bus line in pF. If mix ed with Hs-mode d evices, faste r fall-time s according to Table 6 are

allowed.

n/a = not applicable

PARAMETER SYMBOL STANDARD-MODE FAST-MODE UNIT

MIN. MAX. MIN. MAX.

SCL clock frequency fSCL 0 100 0 400 kHz

Hold time (repeated) START condition.

After this period, the first clock pulse is

generated

tHD;STA 4.0 – 0.6 −µs

LOW period of the SCL clock tLOW 4.7 – 1.3 – µs

HIGH period of the SCL clock tHIGH 4.0 – 0.6 – µs

Set-up time for a repeated START

condition tSU;STA 4.7 – 0.6 – µs

Data hold time:

f or CBUS compatible masters (see NO TE,

Section 10.1.3)

for I2C-bus devices

tHD;DAT

5.0

0(2) –

3.45(3) –

0(2) –

0.9(3) µs

µs

Data set-up time tSU;DAT 250 −100(4) –ns

Rise time of both SDA and SCL signals tr– 1000 20 + 0.1Cb(5) 300 ns

Fall time of both SDA and SCL signals tf– 300 20 + 0.1Cb(5) 300 ns

Set-up time fo r STOP condition tSU;STO 4.0 – 0.6 – µs

Bus free time between a STOP and

START condition tBUF 4.7 – 1.3 – µs

Capacitive load for each bus line Cb– 400 – 400 pF

Noise margin at the LOW level for each

connected device (including hysteresis) VnL 0.1VDD –0.1V

DD –V

Noise margin at the HIGH level for each

connected device (including hysteresis) VnH 0.2VDD –0.2V

DD –V

Philips Semiconductors

The I2C-bus specification

Fig.31 Definition of timing for F/S-mode devices on the I2C-bus.

handbook, full pagewidth

MSC610

SSr tSU;STO

tSU;STA

tHD;STA tHIGH

tLOW tSU;DAT

tHD;DAT

SDA

SCL

tBUF

trtSP

tHD;STA

Philips Semiconductors

The I2C-bus specification

15.2 Hs-mode devices

The I/O levels, I/O current, spike suppression, output slope

control and pin cap ac itan ce for I2C- bu s Hs-mod e devices

are given i n Table 6. The noise mar gin for HIGH and LOW

levels on the bus lines are the same as specified for

F/S-mode I2C-bus devices.

Figure 32 shows all timing parameters for the Hs-mode

timing.The “normal” START condition S does not exist in

Hs-mode. Timing parameters for Address bits, R/W bit,

Acknowle dge bit an d DATA bits are all th e same. Only the

rising edge of the first SCLH clock signal after an

acknow ledge bit has an larger valu e because the external

Rp has to pull-up SCLH without the help of the internal

current-source.

The Hs-mode timing parameters for the bus lines are

specified in Table 7. The minimum HIGH and LOW periods

and the maximum rise and fall times of the SCLH clock

signal determine the highest bit rate.

With an internally generated SCLH signal with LOW and

HIGH level periods of 200 ns and 100 ns respectively, an

Hs-mode master can fulfil the timing requirements for the

external SCLH clo ck pulses (taking the rise and fall times

into account) for the maximum bit rate of 3.4 Mbit/s. So a

basic frequency of 10 MHz, or a multiple of 10 MHz, can

be used by an Hs-mode master to generate the SCLH

signal. There are no limits for maximum HIGH and LOW

periods of the SCLH clock, and there is no limit for a lowest

bit rate.

Timing p arameters are i ndependent for cap acitive load up

to 100 pF for each bus line allowing the maximum possible

bit rate of 3.4 Mbit/s. At a hig her capacitive load on t he bus

lines, the bit rate decreases gradually. The timing

parameters for a capacitive bus load of 400 pF are

specified in Table 7, allowing a maximum bit rate of

1.7 M bit/s. For capacitive bus loads between 100 pF and

400 p F, the timing parameters must be interpolated

linearly. Rise and fall times are in accordance with the

maximum propagation time of the transmission lines

SDAH and SCLH to prevent reflections of the open ends.

Philips Semiconductors

The I2C-bus specification

Table 6 Characteristics of the SDAH, SCLH, SDA and SCL I/O stages for Hs-mode I2C-bus devices

Notes

1. Devices that use non-standard supply voltages which do not conform to the intended I2C-bus system levels must

relate their input levels to the VDD voltage to which the pull-up resistors Rp are connected.

2. Devices that offer the level shift function must tolerate a maximum input voltage of 5.5 V at SDA and SCL.

3. For capacitive bus loads between 100 and 400 pF, the rise and fall time values must be linearly interpolated.

4. SDAH and SCLH I/O stages of Hs-mode slave devices must have floating outputs if their supply voltage has been

switched off. Du e to the current- source o utput cir cuit, wh ich norma lly has a cli pping di ode to V DD, th is re quiremen t is

not mandat or y for the SCLH or the SDA H I/O s tag e of H s-mode mas ter devices . Thi s me ans t hat the sup ply vo ltage

of Hs-mode master devices cannot be switched off without affecting the SDAH and SCLH lines.

PARAMETER SYMBOL Hs-MODE UNIT

MIN. MAX.

LOW level input voltage VIL −0.5 0.3VDD(1) V

HIGH level input voltage VIH 0.7VDD(1) VDD + 0.5(2) V

Hysteresis of Schmitt trigger inputs Vhys 0.1VDD(1) –V

LOW level output voltage (open drain) at 3 m A sink

current at SDAH, SDA and SCLH for:

VDD > 2 V

VDD < 2 V

VOL

00.4

0.2VDD

On resistance of the transfer gate, for both current

directions at VOL level between SDA and SDAH or

SCL and SCLH at 3 mA

RonL −50 Ω

On resistance of the transfer gate between SDA and

SDAH or SCL and SCLH if both are at VDD level RonH(2) 50 – kΩ

Pull-up current of the SC LH current-source. Applies for

SCLH output levels between 0.3VDD and 0.7VDD ICS 312mA

Output rise time (curr en t -s ou r ce enabled) and fa ll time

at SCLH with a capacitive load from 10 to 100 pF trCL, tfCL 10 40 ns

Output rise time (curr en t -s ou r ce enabled) and fa ll time

at SCLH with an ex ternal pul l-up curr ent source of 3 mA

and a capacitive load of 400 pF

trCL(3), tfCL(3) 20 80 ns

Output fall time at SDAH with a capacitive load from 10

to 100 pF tfDA 10 80 ns

Output fall time at SDAH with a capacitive load of

400 pF tfDA(3) 20 160 ns

Pulse width of spikes at SDAH and SCLH that must be

suppressed by the input filters tSP 010ns

Input curr ent each I/O pi n with a n input v o ltage betw een

0.1VDD and 0.9VDD

Ii(4) –10µA

Capacitance for each I/O pin Ci–10pF

Philips Semiconductors

The I2C-bus specification

Table 7 Characteristics of the SDAH, SCLH, SDA and SCL bus lines for Hs-mode I2C-bus devices(1)

Notes

1. A ll values referr ed to VIHmin and VILmax levels (see Table 6).

2. For bus line loads Cb between 100 and 400 pF the timing parameters must be linearly interpolated.

3. A devic e must intern ally prov ide a Data hold ti me to bridge the un defined part be tween VIH and VIL of the fa lling edge

of the SCLH signal. An input circuit with a threshold as low as possible for the falling edge of the SCLH signal

minimi zes this hold time.

PARAMETER SYMBOL Cb = 100 pF MA X. Cb = 400 pF(2) UNIT

MIN. MAX. MIN. MAX.

SCLH clock frequency fSCLH 0 3.4 0 1.7 MHz

Set-up time (repeated) START condition tSU;STA 160 −160 −ns

Hold time (repeated) START condition tHD;STA 160 −160 −ns

LOW period of the SCLH clock tLOW 160 −320 −ns

HIGH period of the SCLH clock tHIGH 60 −120 −ns

Data set-up time tSU;DAT 10 −10 −ns

Data hold time tHD;DAT 0(3) 70 0(3) 150 ns

Rise time of SCLH signal trCL 10 40 20 80 ns

Rise tim e o f SC LH s i gnal after a repeated

START condition and after an

acknowledge bit

trCL1 10 80 20 160 ns

Fall time of SCLH signal tfCL 10 40 20 80 ns

Rise time of SDAH signal trDA 10 80 20 160 ns

Fall time of SDAH signal tfDA 10 80 20 160 ns

Set-up time fo r STOP condition tSU;STO 160 −160 −ns

Capacitive load for SDAH and SCLH lines Cb(2) −100 −400 pF

Capacitive load for SDAH + SDA line and

SCLH + SCL line Cb−400 −400 pF

Noise margin at the LOW level for each

connected device (including hysteresis) VnL 0.1VDD −0.1VDD −V

Noise margin at the HIGH level for each

connected device (including hysteresis) VnH 0.2VDD −0.2VDD −V

Philips Semiconductors

The I2C-bus specification

Fig.32 Definition of timing for Hs-mode devices on the I2C-bus.

(1) First rising edge of the SCLH signal after Sr and aft er eac h ack nowledge bit.

handbook, full pagewidth

MGK871

SDAH

SrSr P

SCLH

= MCS current source pull-up

= Rp resistor pull-up

tfDA trDA

tHD;STA tSU;DAT

trCL

tLOW tHIGH

tHD;DAT

tLOW

tHIGH

trCL1

tfCL

tSU;STO

trCL1 (1)

(1)

tSU;STA

16 ELECTRICAL CONNECTIONS OF I2C-BUS

DEVICES TO THE BUS LINES

The electrical specifications for the I/Os of I2C-bus devices

and t he cha ract eristi cs o f the bu s lines c onnec ted to t hem

are given in Section 15.

I2C-bus devices with fixed input levels of 1.5 V and 3 V can

each have their own appropriate supply voltage. Pull-up

resistors must be connected to a 5 V ± 10% supply

(Fig.33). I 2C-bus devices with input levels related to VDD

must have one common supply line to which the pull-up

resistor is also connected (Fig.34).

When devices with fixed input levels are mixed with

devices with input levels related to VDD, the latter devices

must be connected to one common supply line of

5V±10% and must have pull-up resistors connected to

their SDA and SCL pins as shown in Fig.35.

New Fast- and Hs-mode devices must have supply voltage

related input levels as specified in Tables 4 and 6.

Input levels are defined in such a way that:

•The noise margin on the LOW level is 0.1VDD

•The noise margin on the HIGH level is 0.2VDD

•As shown in Fig.36, serie s resistors (RS) of e. g. 300 Ω

can be used for protection against high-voltage spikes

on the SD A and SCL lines ( result ing from the flash- over

of a TV picture tube, for example).

Philips Semiconductors

The I2C-bus specification

Fig.33 Fixed input level devices connected to the I2C-bus.

handbook, full pagewidth

MBC610

SDA

SCL

RpNMOS

VDD1

RpBiCMOS

VDD2

CMOS

VDD3

BIPOLAR

VDD4

= 5 V 10 %

DD2 - 4

Vare device dependent (e.g. 12 V)

Fig.34 Devices with wide supply voltage range connected to the I2C-bus.

handbook, full pagewidth

MBC625

SDA

SCL

RpRpCMOS CMOS CMOS CMOS

VDD = e.g. 3 V

Fig.35 Devices with input levels related to VDD (supply VDD1) mixed with fixed input level devices

(supply VDD2,3) on the I2C-bus.

handbook, full pagewidth

MBC626

SDA

SCL

RpCMOS

RpCMOS NMOS BIPOLAR

5 V 10 %

VDD1=VDD2 VDD3

VDD2,3 are device dependent (e.g. 12 V)

Philips Semiconductors

The I2C-bus specification

Fig.36 S eri es resistor s (R s) for protection against high-voltage spikes.

handbook, full pagewidth

MBC627

SDA

SCL

RpRp

DEVICE

VDD

I C

RsRs

DEVICE

VDD

I C

RsRs

16.1 Maximum and minimum values of resistors Rp

and Rs for Standard-mode I2C-bus devices

For Standard-mode I2C-bus systems, the values of

resistors Rp and Rs in Fig.33 depend on the following

parameters:

•Supply voltage

•Bus capacitance

•Number of connected devices (input current + leakage

current).

The supply voltage limits the minimum value of resistor Rp

due to the specified minimum sink current of 3 mA at

VOLmax = 0.4 V for the output stages. VDD as a function of

Rp min is shown in Fig.37. The required noise margin of

0.1VDD for the LOW level, limits the maximum value of Rs.

Rs max as a function of Rp is shown in Fig.38.

The bus capacitance is the total capacitance of wire,

connections and pin s. This capa ci tan ce limi ts the

maximum value of Rp due to the specified rise time. Fig. 39

shows Rp max as a function of bus capacitance.

The maximum HIGH level input current of each

input/output connec tion has a specified max imum value of

10 µA. Due t o the r equired noise mar gin of 0.2 VDD for the

HIGH le vel, thi s in put cu rre nt lim it s the ma xi mum val ue of

Rp. This limit depends on VDD. The total HIGH level input

current is shown as a function of Rp max in Fig.40.

Philips Semiconductors

The I2C-bus specification

Fig.37 Minimum va lue of R p as a func tion of supp ly

voltage with the value os Rs as a parameter.

handbook, halfpage

048 16

MBC628

minimum

value Rp

(k )Ω

V (V)

R = 0

max. RS

0 400 800 1600

MBC629

1200

maximum value R (Ω)

15 V

10 V

(k )ΩV = 2.5 V

DD 5 V

Fig.38 Maximum value of Rs as a function of the

value of Rp with supply voltage as a parameter.

Fig.39 Maximum valu e of Rp as a function of bus

capacitance for a Standard-mode I2C-bus.

0 100 200 400

MBC635

300

bus capacitance (pF)

maximum

value R

(kΩ)p

max. R

@ VDD= 5 V

R = 0

Fig.40 Total HIGH level input curren t as a functio n

of the maximum value of Rp with supply vo ltag e as

a parameter.

0 200

MBC630

40 80 120 160

total high level input current (µA)

maximum

value Rp

(k )Ω

5 V

V = 15 V

2.5 V

10 V

Philips Semiconductors

The I2C-bus specification

17 APPLICATION INFORMATION

17.1 Slope-controlled output stages of Fast-mode

I2C-bus devices

The electrical specifications for the I/Os of I2C-bus devices

and t he cha ract eristi cs o f the bu s lines c onnec ted to t hem

are given in Section 15.

Figures 41 and 42 show examples of output stages with

slope c ontr ol in CM OS and bi pol ar tec hnology. The slope

of the fal ling edge is de fined by a Mill er capacit or (C1) and

a resistor (R1). The typical values for C1 and R1 are

indicated on the diagrams. The wide tolerance for output

fall time tof given in Table 4 means that the design is not

critical. The fall time is only slightly influenced by the

external bus load (Cb) and external pull-up resistor (Rp).

However, the rise time (tr) specified in Table 5 is m ainly

determined by the bus load capacitance and the value of

the pull-up resistor.

17.2 Switched pull-up circuit for Fast-mode I2C-bus

devices

The supply voltage (VDD) and the maximum output LOW

level determine th e minimum value of pu ll-up resistor Rp

(see Section 16.1). For example, with a supply voltage of

VDD = 5 V ± 10% and VOLmax = 0.4 V at 3 mA, Rp min =

(5.5 −0.4)/0.003 = 1.7 kΩ. As shown in Fig.33, this value

of Rp limits the maximum bus capacitance to about 200 pF

to meet the maximum tr requirement of 300 ns. If the bus

has a higher capacitance than this, a switched pull-up

circuit as shown in Fig.43 can be used.

The switched pull-up circuit in Fig.43 is for a supply voltage

of VDD = 5 V ± 10% and a maximum capacitive load of

400 pF. Since it is contro lled by the bus levels , it needs no

additional switching control signals. During the

rising/falling edges, the bilateral switch in the HCT4066

switches pull-up resistor Rp2 on/off at bus levels between

0.8 V and 2.0 V. Combined resistors Rp1 and Rp2 can

pull-u p the bus line with in the maxim um specified rise ti me

(tr) of 300 ns.

Series resistors Rs are optional. They protect the I/O

stages of the I2C-bus devices from high-voltage spikes on

the bus lines, and minimize crosstalk and undershoot of

the bus line signals. The maximum value of Rs is

determined by the maximum permitted voltage drop

across this resistor when the bus line is switched to the

LOW level in order to switch off Rp2.

Fig.41 Slope-controlled output stage in CMOS

technology.

MBC618

50 kΩ

2 pF

to input

circuit

VDD

I/O

VSS

VDD

VSS

SDA or SCL

bus line

Fig.42 Slope-controlled output stage in bipolar

technology.

MBC619

20 kΩ

to input

circuit

5 pF

C1 I/O Rp

VDD

VSS

GND

T1 T2

SDA or SCL

bus line

Fig.43 Switched pull-u p circuit.

MBC620

1.3 kΩ

VCC

I/O Cb

VDD

VSS

SDA or SCL

bus line

1/4 HCT4066

nZ GND

nY 5V 10 %

Rp2 1.7 kΩRp1

100 ΩRs

I/O

100 ΩRs

400 pF

max.

FAST - MODE I C BUS DEVICES

Philips Semiconductors

The I2C-bus specification

17.3 Wiring pattern of the bus lines

In general, the wiring must be so chosen that crosstalk and

interference to/from the bus lines is minimized. The bus

lin es are m ost susc eptib le t o cro sstal k and int erfer ence at

the HIGH level because of the relatively high impedance of

the pull-up devices.

If the length of the bus lines on a PCB or ribbon cable

exceed s 10 cm and includes the VDD and VSS lines, the

wiring pattern must be:

SDA

VDD

VSS

SCL

If only the VSS line is included, the wiring pattern must be:

SDA

VSS

SCL

These wiring patterns also result in identical capacitive

loads for the SDA and SCL lines. The VSS and VDD lines

can be omitted if a PCB with a VSS and/or VDD layer is

used.

If the bus lines are twisted-pairs, each bus line must be

twisted with a VSS return. Alternatively, the SCL line can be

twisted with a VSS return, and the SDA line twisted with a

VDD return. In the latter case, capacitors must be used to

decouple the VDD line to the VSS line at both ends of the

twisted pairs.

If the bus lines are shielded (shield connected to VSS),

interference will be minimized. However, the shielded

cable must have lo w capacitive coupli ng between the SDA

and SCL lines to mini mize crosstalk.

17.4 Maximum and minimum values of resistors Rp

and Rs for Fast-mode I2C-bus de vices

The maximum and minimum values for resistors Rp and Rs

connected to a Fast-mode I2C-bus can be determined

from Figs 37, 38 and 40 in Section 16.1. Because a

Fast-mode I2C-bus has faster rise times (tr) the maximum

value of Rp as a function of bus capacitance is less than

that shown in Fig.39 The replacement graph for Fig.39

showing the maximum value of Rp as a function of bus

capacitanc e (Cb) for a Fast-mode I2C-bus is given in

Fig.44.

17.5 Maxim um and minim um values of resist ors Rp

and Rs for Hs-mode I2C -bus de vices

The maximum and minimum values for resistors Rp and Rs

connect ed to a n Hs-m ode I 2C-bus c an be cal cula ted from

the data in Tables 6 and 7. Many combinations of these

values are possible, owing to different rise and fall times,

bus lin e loads, supp ly voltages, m ixed speed sy stems and

level shifting. Because of this, no further graphs are

included in this specification.

18 BI-DIRECTIONAL LEVEL SHIFTER FOR F/S-MODE

I2C-BUS SYSTEMS

Present technology processes for integrated circuits with

clearances of 0.5 µm and less limit the maximum sup ply

voltage and consequently the logic levels for the digital I/O

signals. To interface these lower voltage circuits with

existing 5 V devices, a level shifter is needed. For

bi-directional bus systems like the I2C-bus, such a level

shifter must also be bi-directional, without the need of a

direction control signal(1). The simplest way to solve this

problem is by connecting a discrete MOS-FET to each bus

line.

(1) US 5,689,196 granted; corresponding patent applications

pending.

Fig.44 M ax im um value of Rp as a functi on of bus

capaci tan ce fo r m eeti ng the tr ma x requ irement for

a Fast-mode I2C-bus.

handbook, halfpage

0 100 200 400

7.5

6.0

MBC612

300

4.5

3.0

1.5

bus capacitance (pF)

maximum

value R

(kΩ)p

max. R

@ VDD= 5 V

R = 0

Philips Semiconductors

The I2C-bus specification

In spite of its surprising simplicity, such a solution not only

fulfils the requirement of bi-directional level shifting without

a directi on control sign al, it also:

•isolates a powered-down bus section from the rest of the

bus system

•protects the “lower voltage” side against high voltage

spikes from the “higher-voltage” side.

The bi-directional level shifter can be used for both

Standard-mode (up to100 kbit/s) or in Fast-mode (up to

400 kbit/s) I2C-bus systems. It is not i ntended for Hs-mode

systems, which may have a bridge with a level shifting

possibility (see Section 13.5)

18.1 Connecting devices with different logic levels

Section 16 described how different voltage devices could

be con nected to the same bus by using pu ll-up re sistors to

the supply voltage line. Although this is the simplest

solution, the lower voltage devices must be 5 V tolerant,

which c an make them mor e expensive to manufacture . By

using a bi-directio nal level s hifter, howev er, it’s po ssible to

interconn ect two s ections of a n I2C-bus system, with each

section having a different supply voltage and different logic

levels. Such a configuration is shown in Fig.45. The left

“low-voltage” section has pull-up resistors and devices

connected to a 3.3 V supply voltage, the right

“high -voltage” se ction has pu ll-up resi stors and devices

connected to a 5 V supply voltage. The devices of each

section have I/Os with supply voltage related logic input

levels and an open drain output configuration.

The level shi fter for eac h bus l ine is iden tical and consi sts

of one discrete N-channel enhancement MOS-FET; TR1

for the serial data line SDA and TR2 for the serial clock line

SCL. The gates (g) have to be connected to the lowest

supply voltage VDD1, the sources (s) to the bus lines of the

“lower- voltage” sectio n, and the dr ains (d) to the bus lines

of the “higher-voltage” section. Many MOS-FETs have the

substrate inter nally co nnecte d with its sourc e, if thi s is not

the case, an external connection should be made. Each

MOS-FET has an integral diode (n-p junction) between the

drain and substrate.

Fig.45 Bi-directional level shifter circuit connecting two different voltage sections in an I2C-bus system.

handbook, full pagewidth

MGK879

3.3 V DEVICE 3.3 V DEVICE 5 V DEVICE 5 V DEVICE

TR1

TR2

VDD2 = 5 V

SDA2

SCL2

VDD1 = 3.3 V

SDA1

SCL1

RpRpRp

Philips Semiconductors

The I2C-bus specification

18.1.1 OPERATION OF THE LEVEL SHIFTER

The following three states should be considered during the

operation of the level shifter:

1. No device is pulling down the bus line.

The bus line of the “lower-voltage” section is pulled up

by its pull-up resistors Rp to 3.3 V. The gate and the

source of the MOS- FET are both at 3.3 V, so its V GS is

below the threshold voltage and the MOS-FET is not

conducting. This allows the bus line at the

“higher-voltage” section to be pulled up by its pull-up

resistor Rp to 5 V. So the bus lines of both sections are

HIGH, but at a different voltage level.

2. A 3.3 V device pul ls down the b us line to a LOW lev el.

The source of the MOS-FET also become s L O W,

while the gate stay at 3.3 V. VGS rises above the

threshold and the MOS-FET starts to conduct. The bus

line of th e “higher-vol t age” section is then also pull ed

down to a LOW level by the 3.3 V device via the

conducting MOS-FET. So the bus lines of both

sections go LOW to the same voltage level.

3. A 5 V device pulls down the bus line to a LOW level.

The drain-substrate diode of the MOS-FET the

“lower-voltage” section is pulled down until VGS

passes the threshold and the MOS-FET starts to

conduc t. Th e bus li ne of the “l ower-voltage” s ec tio n is

then further pulled down to a LOW level by the 5 V

devic e via the co ndu cting M OS-FET. So the bus lines

of both sections go LOW to the same voltage level.

The three s tate s show that th e lo gic l evels ar e tran sfer red

in both directions of the bus system, independent of the

driving section. State 1 performs the level shift function.

States 2 and 3 perform a “wired AND” function between

the bus lines of both sections as required by the I2C-bus

specification.

Supply voltages other than 3.3 V for VDD1 and 5 V for VDD2

can also be ap plied, e.g. 2 V for VDD1 and 10 V for VDD 2 is

feasible. In normal operation VDD2 must be equal to or

higher than V DD1 (VDD2 is allowed to fall below VDD1 du ring

switching power on/off).

Philips Semiconductors

The I2C-bus specification

19 DEVELOPMENT TOOLS AVAILABLE FROM PHILIPS

Table 8 I2C evaluation boards

Table 9 Developm ent tool s for 80C51-based syst ems

Table 10 Development tools for 68000-based systems

Table 11 I2C analyzers

PRODUCT DESCRIPTION

OM4151/

S87C00KSD I2C-bus evaluation board with microcontroller, LCD, LED, Par. I/O, SRAM, EEPROM, Clock, DTMF

generator, AD/DA conversion.

OM5500 Demo kit for the PCF2166 LCD driver and PCD3756A telecom microcontroller

PRODUCT DESCRIPTION

PDS51 A board -level, full featured, in-circuit emulator:

RS232 interface to PC, universal motherboard, controlled via terminal emulation

PRODUCT DESCRIPTION

OM4160/2 Microcore-2 demonstration/evaluation board with SCC68070

OM4160/4 Microcore-4 demonstration/evaluation board with 90CE201

OM4160/5 Microcore-5 demonstration/evaluation board with 90CE301

PRODUCT DESCRIPTION

OM1022 PC I2C-bus analyzer with multi-master capability. Hardware and software (runs on IBM or

compatible PC) to experiment with and analyze the behaviour of the I2C-bus (includes

documentation)

OM4777 Similar to OM1022 but for single-master systems only

PF8681 I2C-bus analyzer support package for the PM3580 logic analyzer family

Philips Semiconductors

The I2C-bus specification

20 SUPPORT LITERATURE

Table 12 Data handbooks

Table 13 Brochures/leaflets/lab. reports/books etc.

For more i nformation about Philip s Semicon ductors and how we can help wi th your I2C-bu s desig n, contact yo ur neares t

Philips Semiconductors national organization from the address list of the back of this book, or visit our worldwide web

site at http://www.semiconductors.philips.com/i2c for the latest products, news and applications notes.

TITLE ORDERING CODE

IC01: Semiconductors for Radio, Audio and CD/DVD Systems 9397 750 02453

IC02: Semiconductors for Television and Video Systems 9397 750 01989

IC03: Semiconductors for Wired Telecom Systems (parts a & b) 9397 750 00839,

9397 750 00811

IC12: I2C Peripherals 9397 750 01647

IC14: 8048-based 8-bit microcontrollers 9398 652 40011

IC17: Semiconductors for wireless communications 9397 750 01002

IC18: Semiconductors for in-car electronics 9397 750 00418

IC19: ICs for data communications 9397 750 00138

IC20: 80C51-based 8-bit microcontrollers + Application notes and Development tools 9397 750 00963

IC22: Multimedia ICs 9397 750 02183

TITLE ORDERING CODE

Can you make the distance... with I2C-bus (information about the P82B715 I2C-bus

extender IC) 9397 750 00008

I2C-bus multi-master & single-master controller kits 9397 750 00953

Desktop video (CD-ROM) 9397 750 00644

80C51 core instructions quick reference 9398 510 76011

80C51 microcontroller selection guide 9397 750 01587

OM5027 I2C-bus evaluation board for low-voltage, low-power ICs & software 9398 706 98011

P90CL301 I2C driver ro utines AN94078

User manual of Microsoft Pascal I2C-bus driver (MICDRV4.OBJ) ETV/IR8833

C routines for the PCF8584 AN95068

Using the PCF8584 with non-specified timings and other frequently asked questions AN96040

User's guide to I2C-bus control programs ETV8835

The I2C-bus from theory to practice (book and disk) Author: D. Paret

Publisher : Wiley

ISBN: 0-471-96268-6

Bi-di r ectional level shifter for I2C-bus and other systems AN97055

OM5500 demo kit for the PCF2166 LCD driver and PCD3756A telecom microcontroller 9397 750 00954